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Publishers, New York 

Copyright, 1938, by 
W. W. Norton & Company, Inc. 
70 Fifth Avenue, New York City 

First Edition 

Published by arrangement with 
The University of Chicago Press 

printed in the united states of AMERICA 

This hook is gratefully dedicated to the past and present 
members of our "Ecology Group"; without their enthusi- 
astic co-operation much of the underlying evidence could 
not have been collected during my lifetime, and without 
their critical attention the expression of these ideas 
would have been more faulty. 


Foreword 13 

I. Science versus Metaphysics 15 

II. History and Natural History 20 

III. Beginnings of Co-operation 50 

IV. Aggregations of Higher Animals 90 
V. Group Behavior 133 

VI. Group Organization 175 

VII. Some Human Implications 209 

VIII. Social Transitions 244 

Literature Cited 277 

Index 289 





I A. A hibernating aggregation of ladybird beetles 32 

I B. A breeding aggregation of midges 32 

II. A grassland-bison community 38 

III. Aggregating behavior of brittle starfish 44 

IV. Diagrams showing the effect of population size 

on the rate of evolution 128 

V. Castes of a termite from British Guiana 266 



1. Grasshopper nymphs on the march 36 

2. The effect of numbers present on rate of bio- 

logical processes 52 

3. Group protection from ultra-violet radiation 

for planarian worms 60 

4. Another aspect of group protection for plana- 

rians 62 

5. The small marine flatworm Procerodes 64 

6. Group protection from fresh water for Proce- 

rodes 66 

7. Bacteria frequently do not grow if inoculated 

in small numbers 67 

8. The common sea-urchin Arbacia 70 

9. Arbacia eggs cleave more rapidly in dense 

populations 72 

10. Robertson found that two protozoans placed 

together divided faster than if isolated 76 




11. Other protozoa reproduce more rapidly when a 

certain number of bacteria are present 78 

12. Some protozoans divide more rapidly in dense 

bacterial suspensions if more than one is 
present 79 

13. A and B. A recent suggestion concerning the 

ancestral relations within the animal king- 
dom 86 

14. Goldfish grow more rapidly if placed in slightly 

contaminated water 95 

15. An extract from the skin of goldfish frequently 

has growth-promoting power 97 

16. White mice grow faster in small groups than in 

large ones 101 

17. Flour beetles reproduce more rapidly if more 

than one pair is present 105 

18. The "spread" of time in which eggs are laid in 

a colony of herring gulls affects the per- 
centage that survive ii2 

19. In small populations, genes drift into fixation 

or loss largely irrespective of selection 121 

20. In medium populations complete fixation or 

loss is less likely to occur 123 

21. In large populations, gene frequency is held to 

a certain equilibrium value by the opposing 
pressures of mutation and selection 124 

22. As intensity of selection increases it becomes 

more and more dominant in determining the 
end result 126 

23. Manakin males establish rows of mating courts 

in the Panamanian rain-forest 134 

24. Many kinds of fishes eat more if several are 

present 136 



25. An ant which works at an intermediate rate 

may be speeded up if placed with an ant 
which works more rapidly, or vice versa 141 

26. A simple maze used in training cockroaches 151 

27. Isolated cockroaches make fewer errors during 

training than if paired or if three are trained 
together 152 

28. They also take less time 153 

29. Parrakeets learn equally well if trained when 

isolated, whether they are caged singly or in 
pairs 156 

30. Parrakeets learn more rapidly if trained alone 

than if two are placed together in the maze 157 

31. Feeding a fish which has just come through the 

opening from the larger side of the aquarium 160 

32. Goldfish learn to swim a simple aquarium-maze 

the more readily the more fish there are 
present 161 

33. Isolated goldfish learn the problem set for them 

less rapidly, and unlearn it more readily 162 

34. The aquarium-maze used in training part of 

the fish to come forward and part to go to the 
rear to be fed 164 

35. Cyprinodon learn to move in a body more read- 

ily than to split into two separate groups 165 

36. Goldfish learn more readily if accompanied by 

a trained leader 166 

37. An aquarium-maze arranged to test the power 

of observation of fish 168 

38. Goldfish react more rapidly if allowed to watch 

others perform ^ 169 

39. Flocks of hens are organized into a definite so- 

cial hierarchy 178 

40. Cockerels also have a social organization 180 



41. In flocks of pigeons the organization is one of 

peck-dominance rather than of peck-right 187 

42. The Dionne quintuplets also show evidence of 

a social organization among themselves 204 

43. The percentage of births that were canceled by 

deaths for the given years in Italy and Ger- 
many 220 

44. The percentage which deaths were of births 

steadily increased during the war years 223 

45. Crepidula fornicata shows sex reversal 254 

46. Mated males of Crepidula fornicata retain that 

stage longer 256 

47. Castes of the common honey-bee 260 

48. Some ant castes 265 

49. The brown locust of South Africa has a swarm 

phase which is distinct from the solitary 
phase 273 


I WAS recently honored by an invitation to give the 
Norman Wait Harris lectures at Northwestern Uni- 
versity; the more so since as one of their side-door 
neighbors I live close enough for my personal foibles 
to be well known, thereby removing the chief source 
of any possible glamour. In this book which grew out 
of those lectures, as in the lecture series itself, I make 
no effort to pose as the remote purveyor of a mys- 
terious erudition; I could not in any case regard my- 
self as more than the exponent of the glorified com- 
mon sense which I more and more firmly believe all 
science should be. 

Even more than most, this book is the outgrowth 
of years of co-operative effort. Some of the basic 
facts were collected with the aid of funds from the 
Rockefeller Foundation given to aid biological re- 
search at the University of Chicago. Other researches 
were supported directly by that university and more 
recently by a grant for the study of the effect of 
hormones on behavior from the National Research 




In addition to the personal aid received from my 
scientific associates, many of whom will be named in 
the text, the kindly criticism of Professor Alfred E. 
Emerson has been particularly helpful in developing 
the work and in shaping the content and implica- 
tions of these lectures. My thanks are given also to 
Professor Sewall Wright for his criticism of Chapter 
IV, to Mr. Kenji Toda for preparing the illustra- 
tions and to Marjorie Hill Allee, whose command of 
the written word has been a constant resource. 

W. C. Allee 
The University of Chicago. 

Mm Science versus Metaphysics 

THE RATE of obsolescence of material things is 
high. With consumers' goods we are well aware of 
this fact; and even capital goods usually become out 
of date in a long generation. Last summer an admirer 
of Will Rogers dedicated a lasting monument to the 
humorist. Although built for time and erected in our 
semi-arid West where decay is slow, the tower is ex- 
pected to last only a thousand years. Invested capital 
evaporates even with watchful care; there are few 
private collections of material wealth that remain in- 
tact a third of a thousand years. 

Oddly enough, the most permanent contributions 
of our age appear to be the scientific discoveries we 
have made, the artistic beauties we have created, and 
the ideas we have evolved. To the extent that these 
advances are entombed in libraries and museums 
they share the impermanence of more material 
things. A nearer approach to immortality is per- 
mitted those bits of science and art that escape from 
the bindings of books and pass into the active life 



and traditions of people. The more widespread and 
firmly fixed these become in the minds of living men, 
the greater is their chance of longevity. 

The most practical achievement of our extremely 
practical period is the habit of searching for new 
truths and for correct interpretations of those long 
known. The unique contribution of the present era 
is not that made by men of business and affairs, spec- 
tacular as it may be. Rather this age is and will 
be known as the time of the development and ap- 
plication of scientific methods. These contributions 
are being made by extremely impractical research 
workers who are supported by a tiny splinter from 
the great block of capital gains. Money spent effec- 
tively to this end, whether in the aid of research or 
other creative scholarship, or in teaching the results 
gained by research, makes the most lasting and im- 
portant of all modern investments. The most nearly 
permanent monument any man can erect is to have 
influenced directly or indirectly the growth of im- 
proved ideas and traditions among the men in the 
street, in the factory or on the farm. 

It is in this spirit that I have undertaken to inter- 
pret one of the significant biological developments 
of recent years. It is my hope that from the work 
described in these pages, all social action may have a 
somewhat broader and more intelligent foundation. 


We can gain the impression from some modern 
oversimplifications that science deals with empirical 
facts, that philosophy attends to principles and 
eternal truths, and that religion is concerned with 
values. In the following pages it will be necessary 
to shake aside such artificial limits and to present 
principles along with the evidence that supports 
them; to test these against experience and to attempt 
frequently to weigh the general biological values 
involved. This last process will be easier if we assay 
survival values only. Admittedly in dealing even 
with survival values we must be relatively rough 
and ready in our methods, and perhaps the conclu- 
sions will carry a strong odor of the laboratory in 
which they had their origin. 

Basically the approach will be that of the experi- 
mental biologist rather than that of the theorist, 
which might be more polished, or of the philoso- 
pher, which would certainly be more abstract and 
would probably use a great many more words for 
the same number of ideas. Despite much practice 
to the contrary, any biological fact which concerns 
us can be accurately described and the conclusions 
from its study be clearly expressed in relatively sim- 
ple and direct language. 

In research reports and scholarly discussions there 
is need for the conciseness and precision made pos- 


sible by technical language. Science has no need, 
however, and is ill-served by any tendency to de- 
velop a cult of obscurity. Scientists must be free to 
attack the unknown as effectively as they can and 
in return for intellectual freedom they have an 
obligation, which rests heavily on those able to do 
so, to interpret research results in terms which can 
be understood by intelligent and interested people. 
There is current in at least one American uni- 
versity at present an attempt to organize all knowl- 
edge about metaphysics, and so secure a longed-for 
unity. In order to obtain a simplified system, the 
group of men occupied with this enterprise turn 
back to the days before the present scientific era to 
find a statement of eternal principles which will 
serve as a unifying nucleus for human experience 
and thought. Such efforts at establishing a Neo- 
Scholastic philosophy, while furnishing an excellent 
corrective for overconfident scientists, seem mis- 
chievously naive as a serious, present-day movement. 
We do need relief from our absorbed attention to 
conflicting scientific detail, but progress must needs 
come from newer syntheses which take account of 
the world and man as science sees them rather 
than by accepting almost as a whole some ancient 
system of historical significance. These systems are out 
of date primarily because they were evolved before 


one of the greatest advances in knowledge that man 
has yet been able to make, that of modern science. 

Modern philosophical educational systems, if they 
are to survive, must have as their central core the 
well-tested evidence compiled by objective scientific 
methods. Such knowledge must have stood the test 
of being checked and re-checked by men constitu- 
tionally agnostic in their mental attitudes; who can 
say, "I don't know. What is the evidence?"; who are 
constantly seeking critical new evidence concerning 
the validity of their ideas. 

An anecdote that is becoming classic among scien- 
tists will illustrate the point. Professor Wood, phys- 
icist of Johns Hopkins, was asked to respond to the 
toast "Physics and Metaphysics" at a dinner of some 
philosophical society. His response was somewhat as 

The physicist gets an idea which seems to him to 
be good. The more he mulls over it the better the 
idea appears. He goes to the library and reads on 
the subject and the more he reads the more truth 
he can see in his idea. Finally he devises an experi- 
mental test and goes to his laboratory to apply it. 
As a result of long and careful experimental check- 
ing he discards the idea as worthless. "Unfortu- 
nately," Professor Wood is said to have concluded, 
"the metaphysician has no laboratory." 

History and Natural History 

LIKE other human disciplines, science has its or- 
thodox and its heterodox views. The idea that un- 
conscious automatic co-operation exists has had a 
long history, and yet it is just now beginning to 
escape from the heterodox category. 

My own interest in this subject dates not from a 
preconceived idea but from a clearly remembered 
bump against some stubborn experiments. Almost 
thirty years ago as a graduate student in zoology I 
was engaged in studying the behavior of some com- 
mon small fresh-water animals called isopods, tiny 
relatives of the crayfish. All fall and winter I had 
been collecting them from quiet mud-bottomed 
ponds, chopping the ice if necessary, and from be- 
neath stones and under leaves in clear small streams. 

I kept them in the laboratory under conditions 
which resembled those in which they lived in na- 
ture. Then day after day I put lots of five or ten 
isopods into shallow water in a round pan that had 
a sanded wax bottom so prepared that the isopods 



could crawl about readily. When a current was 
stirred in the water the isopods from the streams 
usually headed against it; but those from ponds were 
more likely to head down current, or to be indif- 
ferent in their reaction to the current. The behavior 
of the two types was sufficiently different so that at 
first I thought that stream and pond isopods repre- 
sented different species, but the specialist at the 
National Museum assured me that all belonged to 
the species appropriately called Asellus communis, 
the commonest isopod of our inland waters. 

Rather cockily I reported after a time to my in- 
structor that I had gained control of the reaction of 
these animals to a water current. By the judicious 
use of oxygen in the water, I could send the indif- 
ferent pond isopods hauling themselves upstream, or 
I could reduce the stream isopods to going with the 
current. I had not reckoned with another factor that 
presently caught up with me. 

After a winter in the laboratory it seemed wise as 
well as pleasant to take my pan out to a comfortable 
streamside one sunny April day, and there check the 
behavior of freshly collected isopods in water dipped 
from the brook in which they had been living. To 
my surprise, the stream isopods, whose fellows all 
winter had gone against the current, now went 
steadily downstream or cut across it at any angle to 


reach another near-by isopod. When I used five or 
ten individuals at a time, as I had done in the labo- 
ratory, they piled together in small close clusters 
that rolled over and over in the gentle current. 
Only by testing them singly could I get away from 
this group behavior and obtain a response to the 
current; and even this reaction was disconcertingly 

It took another year of hard work to get this con- 
tradictory behavior even approximately untangled; 
(i) * to find under what conditions the attraction of 
the group is automatically more impelling than keep- 
ing footing in the stream; and that was only the 
beginning of the road that I have kept from that 
April day to this time, continuing to be increasingly 
absorbed in the problems of group behavior and 
other mass reactions, not only of isopods, but of all 
kinds of animals. 

As the years have gone on, aided by student and 
other collaborators and by the work of independent 
investigators, I have tried to explore experimentally 
the implications of group actions of animals. And 
necessarily, too, I have had to turn to the world's 
literature to find what others have done and are 
doing along this line. 

* Detailed citations to more complete statements will be found in 
the bibliography. 


A Greek philosopher named Empedocles seems 
to have had the first recorded glimmerings of an 
idea of the universal and fundamental nature of 
co-operation which underlies group action, as well 
as a conception of the opposing principle of the 
struggle for existence. Empedocles lived in the fifth 
century B.C., and he was not only a thinker but so 
much a man of affairs that he was offered a king's 
crown, which he refused. (128) 

He owes his present-day fame to two long poems 
in which he outlined the idea that there are natural 
elements: fire, earth, air, and water, which are acted 
upon by the combining power of love and the dis- 
rupting power of hate. Under the guidance of the 
building force of love the separate elements came 
together and formed the world. Separate parts of 
plants and various unassorted pieces of animals arose 
from the earth. These, Empedocles taught, were 
often combined and at first the results were mon- 
strous shapes, which in time became straightened 
around until, still guided by combining love, they 
clicked, to make the more perfect animals we now 
know. It has taken us almost two and a half mil- 
lennia to transmute this poetic conception into the 
less picturesque but more exact and workable ex- 
pression acceptable to modern science. 

After the fertile Greek era there intervened in this 


field as elsewhere the long sterile period when Greek 
philosophy, if known, was dogmatically accepted, 
and shared with other authoritarian systems the re- 
sponsibility of explaining the world of reality as well 
as the universe of fancy. 

It was not until my own experiments and think- 
ing and reading had begun to form in my mind a 
fairly definite pattern that, by the aid of Havelock 
Ellis's The Dance of Life (43) I stumbled upon the 
ideas of the third Earl of Shaftesbury, who lived be- 
fore and after 1700. He seems to have been the first 
intellectual in the modern period to recognize fairly 
clearly that nature presents a racial impulse that has 
regard for others, as well as a drive for individual 
self-preservation; that, in fact, there are racial drives 
that go beyond personal advantage, and can only be 
explained by their advantage to the group. 

An unfriendly contemporary wrote pretty much 
these words: "Shaftesbury seems to require and ex- 
pect goodness in his species as we do a sweet taste in 
grapes and China oranges, of which, if any are sour, 
we boldly proclaim that they are not come to their 
accustomed perfection." Havelock Ellis, in reviewing 
this development, says that "therewith 'goodness* 
was seen practically for the first time in the modern 
period to be as 'natural' as the sweetness of ripe 
fruit." It is only fair to record that in the religious 


world for at least fifty years previous there had been 
growing a similar conviction among certain heretics. 

In 1930, after having written the text of a care- 
ful account of experimental evidence concerning the 
existence and non-existence of co-operation at sub- 
social levels, (3) I set down in the draft of a proposed 
preface that the existence of such a principle was 
now for the first time an established fact, for which 
the details to follow gave full proof. I still think 
that the proof is good. However, the preface as 
published does not contain any such claim, for at 
that point in the writing I went back and re-read 
Des societes animales by the French scientist Es- 
pinas, (44) which appeared in 1878 and which was 
the pioneering essay in this field so far as modern 
work is concerned. There I found Espinas affirming 
that no living being is solitary, but that from the 
lowest to the highest each is normally immersed in 
some sort of social life, a fact which he proclaimed 
sixty years ago, and added that he was ready to offer 
conclusive proof. 

I turned through the pages past his detailed his- 
tory of the evolution of ideas about the origin and 
development of society, and read his massed evi- 
dence that communal life is not "a restricted acci- 
dental condition found only among such privileged 


species as bees, ants, beavers and men, but is in fact 

The evidence was largely based on observations 
of the existence of animal groupings in nature, 
which are found widely distributed in the different 
levels of the animal kingdom— facts such as I shall 
review later in this chapter. It was clear to me that 
the facts which Espinas had found so impressive had 
not convinced others and, while suggestive, did not 
seem compelling to me in the light of other indica- 
tions to the contrary. Perhaps, I cautioned myself, 
even the experimental evidence that I had accumu- 
lated in 1930 was not really crucial, and it would 
be well to avoid making too strong a claim in the 
matter. The same caution must continue even in the 
face of still stronger evidence known today. 

The conclusions of Espinas coming in 1878 are 
the more important because the scientific world was 
then, as men in the street are today, under the spell 
of the idea that there is an intense and frequently 
very personal struggle for existence so important and 
far reaching as to leave no room for so-called softer 
philosophies. The idea of the existence of natural 
co-operation was apparently in the air despite the 
preoccupation with this phase of Darwinism. Kessler 
is said to have addressed a Russian congress of natu- 
ralists on this subject in 1880, and from this ad- 


dress sprang the remarkable if uncritical book by 
the Russian anarchist, Prince Kropotkin, on mutual 
aid. (74) 

By combing the accumulated natural history rec- 
ords, Kropotkin was able to collect observation after 
observation which indicated that animals in nature 
do aid each other to live, as well as, on occasion, kill 
each other off. Kropotkin's work served the admi- 
rable purpose of keeping this idea alive and popu- 
larizing it. It has had also the less fortunate result 
of bringing Kropotkin's fundamental doctrine into 
disrepute among students who are critically sensi- 
tive to the value of evidence, and who find that 
Kropotkin's sources were not always reliable. 

William Patten, an American biologist who taught 
for many years at Dartmouth College, made the next 
general statement of the fundamental nature of co- 
operation when in 1920 he gave it a central place 
in his analysis of the grand strategy of evolution, (go) 
It is of personal interest to me that at the scientific 
meetings in 1919 at which I presented my first ex- 
perimental results on this subject, Professor Patten 
gave a vice-presidential address in which he outlined, 
mainly from philosophical considerations, his con- 
clusions concerning the importance of biological 
co-operation. He was rightly impressed by the fact 
that cells originally were separate, as protozoans are 


today. Some, however, evolved the habit of remain- 
ing attached together after division. This made a 
beginning from which the many-celled higher ani- 
mals could develop. With each increase in the ability 
of cells to co-operate together there came power to 
increase the complexity of organization of the cell 
masses. The highly evolved bodies of men and of 
insects are thus an expression of increasing inter- 
cellular co-operation which finally reaches a point 
at which, for many purposes, the individual person 
becomes the unit rather than the co-operating cells 
of which he is composed. 

About the same time the German, Deegener, (40) 
published an extensive treatise on the social life of 
animals, along the same lines as the book written 
by Espinas forty years before. Deegener 's distinctive 
contribution was a classification of the different 
social levels, from the simplest sorts of artificial col- 
lections of animals to parasitism and truly social 
life. His rating of these different aspects of sub-social 
and social life in one long outline has the great 
merit of showing that there are no hard and fast 
lines which can be drawn between social and sub- 
social organisms, but that social communities are 
the natural outgrowth of sub-social groupings. Un- 
fortunately, wdth Teutonic vigor and vocabulary, 
he designated the different categories in words as 


unwieldy as they were exact. Bogged down by the 
weight of such terms as sympatrogynopaedium, syn- 
aporium and heterosymphagopaedium, Deegener's 
real contribution tends to be lost even to biological 

A survey such as I am attempting here should not 
try to be exhaustive; I shall dismiss with a word the 
slight advance made by Alverdes (16) and the work 
of many others without that. There is, however, 
another phase of the literature whose reading has 
given me so much pleasure as well as useful infor- 
mation that I shall not pass it over: this deals with 
the social insects. Espinas, Kropotkin, Deegener and 
Alverdes of those mentioned, and a host of others, 
have written in detail and in general about these 
fascinating insects, but none more accurately or 
with greater insight and literary as well as scientific 
skill than the American entomologist, William 
Morton Wheeler. His book on Social Life Among 
the Insects, which appeared in 1923, is a noteworthy 
general summary. (120) In this he shows that among 
insects alone, and including such well-known forms 
as termites, bees, wasps and ants, and the less gen- 
erally known social beetles, the social habit has 
arisen some twenty-four distinct times in about one- 
fifth of the known major divisions of insects. It 
would seem that there is a general reservoir of pre- 


social traits from which, given the proper opportu- 
nity, society readily emerges. Wheeler, no less than 
Espinas, from whom he quotes, emphasizes that even 
so-called solitary species of animals are of necessity 
more or less co-operative members of associations of 
animals and that animals not only compete among 
themselves but they also co-operate with each other 
to secure mates and insure greater safety. 

It did not, however, make for the full acceptance 
of these ideas that Wheeler drew his illustrative 
material primarily from, and based his conclusions 
mainly on, his knowledge of social life among in- 
sects. The existence of co-operation among nest 
mates in ants and bees does not prove that there are 
beginnings of co-operative processes among amoebae 
and other greatly generalized animals. 

Man and the few species of highly social insects 
are a small part of the animal kingdom; in order 
to discover and distinguish the principles of general 
sociology it is necessary to look farther, to focus 
attention on the social and anti-social relationships 
of many animals usually regarded as lacking social 

With and without this end in view there have 
been in the last twenty years simultaneous but inde- 
pendent outbreaks of experimentation on group 
effects among the lower animals. For a time just 


preceding and following 1920 we, who in Aus- 
tralia, (107) in France (26) and in the United 
States (2) were engaged in these studies, continued 
unaware of each other's work. Relatively soon, how- 
ever, since biological world literature is today widely 
and promptly circulated, all such work, even that 
in Russia, (53) became generally known. It is these 
general experiments on population growth, on mass 
physiology and on animal aggregations, that are now 
the important aspect of the field of animal co- 

I have briefly traced here the history of the idea 
of innate co-operation. One reason for the slowness 
of accepting that idea is the obvious fact that co- 
operation is not always plain to the eye, and that 
competition in its most non-co-operative form, in 
which no social values are apparent, can readily be 
observed. With certain exceptions to be nientioned 
soon, it has seemed that, social species aside, crowd- 
ing, the simplest start toward social life which is 
easily apparent and a condition of nearly all society, 
was harmful alike to the individual and to the race. 
It has been known from experimental evidence 
since 1854 (62) that crowded animals may not grow 
at all, or, at any rate, gi-ow less rapidly than their 
uncrowded brothers and sisters. And under many 
conditions crowded animals not only do not grow. 


they die more readily, and frequently they repro- 
duce less rapidly than if living in uncrowded popu- 

All the older works in natural history taught 
fairly clearly that crowded groups, to have real sur- 
vival values, must be sufficiently well organized to 
contribute to group safety by warning of danger or 
by defense in case of attack. (3) If, in addition, these 
groups are organized on a basis of division of labor, 
such as occurs in the highly social colonies of ants 
or termites, with specialized reproductives, workers 
and soldiers, or according to the patterns found in 
human society, then the survival values of groups 
are readily seen. 

Yet for some reason, under natural conditions and 
with very many sorts of animals, crowding in all 
degrees does occur and apparently always has oc- 
curred. Conceded that animals do not always act for 
their own best interests, still they must do so to a 
certain degree or be exterminated in the long run. 
The advantages of the long-established habit of a 
species may not be obviously apparent, but it is not 
safe to say offhand that advantages do not exist. 

There are the dense crowds of certain animals, 
ladybird beetles (Plate la), for example, that with 
the approach of winter collect in restricted and fa- 
vorable places where they hibernate together. Ap- 

PLATE I. a. Ladybird beetles cellect in dense ag- 
gregations in the autumn and hibernate. /;. During 
their breeding season, male midges gather in swarms 
and await the coming of .the females. (Photographs by 


parently, in the face of winter cold, there is some 
safety in numbers even among cold-blooded animals 
that collect in hordes without any organization. 

A second plain exception to the general testimony 
that crowding of non-social species is harmful are 
the aggregations that form during the breeding sea- 
son. Like the hibernating groups, these are very 
widely distributed through the animal kingdom. 
Breeding aggregations of worms, crustaceans, fishes, 
frogs, snakes, birds and mammals or the midge in- 
sects shown in Plate lb, for example, have long at- 
tracted attention. Their numbers have been great 
enough and conspicuous enough to stimulate re- 
peated descriptions by naturalists. 

A third exception is found during times of migra- 
tion, when animals frequently crowd together in 
great hordes and execute mass migratory movements, 
like those of many birds. 

However, breeding, hibernation and migration 
aside, the older information indicated that up until 
the point that social life is developed, crowding is 

But there are many other instances of crowding 
which do not fall under any of these classifications; 
and it will be worth while to consider here the ex- 
tent and the natural history of some of these dense 
animal aggregations. Here, as elsewhere, there will 


be no attempt to catalogue all known instances or 
to select merely the very best cases known. I shall 
try to use examples that are not too shopworn by 
repeated description. 

Almost every observant person has seen the soft 
green "bloom" which covers many stagnant ponds. 
Under the microscope this "bloom" is often seen to 
be composed of myriads of the tiny plant-animal 
Euglena. These organisms are commonly one-tenth 
of a millimeter long, which means that in a char- 
acteristic layer of "bloom" there would be at least 
sixty to one hundred thousand animals per square 
inch; and acres of water are sometimes covered. 

Lobster-krills are small crustaceans that occur com- 
monly in shoals about the Falkland Islands, Pata- 
gonia, New Zealand and other southern waters. (81) 
A larval stage of this animal, less than an inch long, 
occurs often on the surface of the water in such 
numbers that the sea is red for acres; and whales in 
those waters simply open their mouths and swim 
through slowly, feeding with no more effort than 
the process of straining them out. These shrimp- 
like animals may be piled up on the shore by tide 
and wind in stench-producing layers. Dampier wrote 
of them in 1700: "We saw great sholes of small lob- 
sters, which colored the sea red in spots for a mile 
in compass"; and they have been known to extend 


along the Patagonian coast for as much as three hun- 
dred miles. 

At Woods Hole, on Cape Cod, I have at certain 
seasons dipped up a bucket of sea water from the 
harbor and found more space occupied by clear, 
jelly-like ctenophores, each the size of a walnut, 
than was taken by water. Sometimes I have dipped 
up a fingerbowl of sea water and found it so filled 
with small pin-point-like copepods that again there 
seemed to be more of them than of the water itself. 
These tiny relatives of the lobster-krills are also the 
food of whales, and they, too, may discolor the 
ocean for miles. 

Around bodies of fresh water, may-flies or midges 
may emerge in clouds. At Put-in-Bay, near the 
lights flooding the monument that commemorates 
Perry's victory, I have picked up living may-flies by 
the double handfuls from the millions that fly to- 
ward the lights; and near by our lake boat steamed 
through windrows of cast skins of the emerging may- 
fly nymphs. Nearer Chicago I have taken water 
isopods, the half-inch crustaceans mentioned earlier, 
by the bucketfuls from pools where they had col- 
lected in numbers only to be compared with those 
in twenty swarms of bees. 

We have already spoken of the migratory hordes. 
Locusts in migration (116) swarm out of the sky in 


the Sahara borderlands, in southern Russia, in 
South Africa and on the Malay Peninsula in ter- 
rorizing numbers (Figure 1). They once did so on the 
Great Plains of the United States, leaving a lively 
memory of destruction that is still roused by the 
smaller migrations that may occur there any summer 

^''' ^ ■.■:,:■:. . ■■ . -•■•■■■•• \ 

Fig. 1. A band of grasshopper nymphs on the march. 
(From Uvarov, by permission of the Imperial Bureau of 

in spite of active control measures. I myself have 
seen the so-called Mormon cricket advancing from 
the relatively barren mountain pastures of Utah 
into the green fields in numbers which were not 
halted by the hawks, turkeys and snakes attendant 
on the swarm and feeding greedily; or the active 
assaults of men and children warned out to protect 
the cultivated lands. Migrating army worms and 
chinch bugs present equally impressive aggregations. 
The emergence of Mexican free-tailed bats from 
the Carlsbad cave of an August evening has been 
described as a black cloud pouring out in such den- 
sity as to be visible two miles away. (19) Such bats 


are estimated to hibernate in these caves by the 
milHons; and they may be found through the day 
in sleeping masses a yard across, hanging from the 
roof like a swarm of bees. 

Even larger mammals may collect into great, 
closely packed herds. The migrating caribou on 
the tundra are said to pour south in hordes that 
flow past a given point for hours or even for days. 
And of the antelope on the plains of Mongolia, (17) 
Roy Chapman Andrews says that he has seen thou- 
sands upon thousands of bucks, does, and fawns 
pour over the rim and spread out on the plain. 
Sometimes a thousand, more or less, would dash 
away from the fierd, only to stop abruptly and feed. 
The mass of antelope were in constant motion even 
when the animals were undisturbed. They scattered 
before his automobile only to re-form within a few 
hours. In that region only the grassland antelope 
gathers in such immense herds; the long-tailed 
desert species never does so, probably because there 
is not enough food to support them in their more 
arid dwelling place. 

These are merely a few of the more dramatic 
instances of the collection of great masses of animals 
in a small space. They are more spectacular but 
probably less important than are the innumerable 
smaller aggregations of animals which are frequently 


encountered. The small dense crowds of whirligig 
beetles are a case in point. These occur in wide- 
spread abundance on the surface of our inland 

The more common condition of less intense crowd- 
ing does not mean that animals are usually solitary. 
Rather, the growing weight of evidence indicates 
that animals are rarely solitary; that they are almost 
necessarily members of loosely integrated racial and 
interracial communities, in part woven together by 
environmental factors, and in part by mutual attrac- 
tion between the individual members of the different 
communities, no one of which can be affected with- 
out changing all the rest, at least to some slight 

Let us take an example. Before the coming of the 
white man, and even a century ago or less, much of 
the Great Plains was occupied by what ecologists 
call a grassland-bison community. (4) Grasses could 
readily grow in the rich soil, even with the usual 
summer dry spells and the more severe cyclic 
drouths that occurred even then. By keeping the 
grasses fairly closely cropped the bison herds pre- 
vented the invasion of herbs and shrubs that might 
have withstood the severities of the climate but 
could not make headway against continual grazing 
(Plate II). In this function the bison were joined by 

PLATE II. A giasslancl-bison community. (Photo- 
graph from the National Park Board of Canada.) 


a myriad of grasshoppers, crickets, meadow mice 
and prairie dogs. All these were key-industry ani- 
mals. In one way or another they converted the grass 
into meat of different sorts, on which the plains 
Indians, buffalo wolves, haw^ks, owls, and prairie 
chickens fed. If the grass failed, then many of the 
key-industry herb-eaters and those that in turn fed 
on them must either starve, migrate into another 
community where they would be disturbing factors, 
or change their source of food and thereby disturb 
the balance in their own community. 

It must be pointed out here that the plants of this 
community cannot be set off as separate from the 
animals. They divide the available space between 
them; they constantly interact upon each other and 
upon their physical environment; except for pur- 
poses of formal study or in limited fields, the biolo- 
gist must consider both as members of a given 

In such a community the effects of the dominant 
bison were felt in times of stress by the humblest 
and least conspicuous grasshopper. In the spring of 
the year hundreds of square miles normally sup- 
ported populations of six to ten million insects and 
other invertebrate animals for every acre of land. 
As with warmer weather the predatory animals re- 
turned to the grasslands, these insects were eaten off 


until perhaps a tenth of their number could be 
found later in the season; with the autumn lushness 
they increased again, only to fall back to some half- 
million or so per acre during the winter cold. 

Similar communities exist among aquatic forms. 
In fact one of the first demonstrations of such a 
community was made for the animals living in and 
on an oyster-bank. (82) A beautiful and penetrating 
description of the interrelations that may be found 
in a small lake was published not long after by the 
late Professor Forbes (48) of the Illinois Biological 
Survey, in which he pointed out that minnows com- 
peted with bladderwort plants for key-industry or- 
ganisms; and showed that when a black bass is 
hooked and taken from the water the triumphant 
fisherman is breaking, unsensed by him, myriads of 
meshes which have bound the fish to all of the dif- 
ferent forms of lake life. 

The existence of these communities is now gen- 
erally recognized, and in order that they may exist 
it seems that there must be a far-reaching, even if 
vague and wholly unconscious, co-operation among 
all the living creatures of the community. It is to 
such relationships that Wheeler referred when he 
said, "Even the so-called solitary species are neces- 
sarily more or less co-operative members of groups 
or associations of animals of different species." 


Within these communities aggregations of animals 
occur for a variety of reasons. Their nature can best 
be shown by a series of illustrations. 

One variety of aggregations is that of colonial 
forms, in which many different so-called individuals 
remain through life permanently attached together. 
In the simplest cases all the individuals are alike. 
Each possesses a mouth and food-catching tentacles, 
and each feeds primarily for itself, although food 
caught by one individual may be shared with others 
near by. In more complex forms some individuals 
have the mouths suppressed, and receive all their 
food from those that do take food. They have be- 
come specialized as bearers of batteries of stinging 
cells; they strike actively when the colony is touched, 
and their stinging cells explode so effectively as to 
give protection to the colony. Other individuals in 
the same colony bear medusa-like heads which break 
away and swim off, producing eggs and sperm, dis- 
tributing them as they drift. Here is certainly a divi- 
sion of labor though these colonial animals are 
never rated as social. 

Various modifications of such colonial animals 
are found particularly among the colonial protozoa, 
sponges and the coelenterates; they also occur higher 
in the animal kingdom, even among the lower 
chordates, the great phylum to which man himself 


belongs. It is interesting that animals whose struc- 
ture forces them to the sort of compulsory mutual 
aid that automatically follows such structural con- 
tinuity have never progressed far either in social 
achievement or in the evolutionary scale. When 
higher animals, such as the lower chordates, show 
this development they are usually regarded as de- 
generate members of their general stock. These 
colonial animals are seldom dominant elements in 
the major communities of which they are a part. 
One comes to the conclusion that the more nearly 
voluntary such co-operation is, the greater its ad- 
vantage in social life. It might on the other hand 
be pointed out that when an animal has achieved 
social organization and division of labor low in the 
evolutionary scale, the resulting colonies are so well 
adapted to their environment that there is not suffi- 
cient pressure to cause evolutionary changes. 

A second type of aggregation occurs when animals 
are forced together willy-nilly by the action of wind 
or tidal currents or waves over which they have no 
control, and whose effects they cannot resist. Many 
of the masses which lend color to wide patches of 
the ocean surface are brought together by tempo- 
rary or permanent currents. Often animals so dis- 
tributed are thrown down more or less by chance 
on types of bottom on which they can develop, and 


there, if favorable niches are somewhat rare, dense 
aggregations may result, like New England coral on 
a suitably hard bottom, or the animals found on a 
wharf piling. 

These accidental animal groupings may persist 
only as long as the physical forces which brought 
them together continue to act. Usually, however, 
they last somewhat longer, as a result of a slightly 
positive social inertia which tends to keep animals 
concentrated in whatever place they happen to be 
found. If the groupings are to have much perma- 
nence this quality of social inertia, the tendency of 
animals to continue repeating the same action in 
the same place, must be reinforced by another 
quality: the social force of toleration for the pres- 
ence of others in a limited space. The densely packed 
communities of animals on a wharf piling can per- 
sist only if toleration for crowding is well developed. 

Other dense collections may be brought about by 
forced movements of animals in response to some 
orienting influence in their environment. These 
oriented, compelled reactions are frequently called 
tropisms. They are shown by the moths or June 
beetles or may-flies that collect about lights. Such 
aggregations are a result of the inherited, internal 
organization of the animals; and the irresistible at- 
traction of the may-fly to the light is joined with 


active toleration for the close proximity of others. 

Similarly close aggregations occur as a result of 
the less spectacular trial and error reactions, in 
which the animals wander here and there, more or 
less vaguely stimulated by internal physiological 
states or external conditions, and so come to collect 
in favorable locations. Collections of animals about 
limited sources of food give a good illustration. 
These, too, may show only the social qualities of 
inertia and toleration. 

A decided advance is made when animals react 
positively to each other and so actively collect to- 
gether, not primarily because the location is favor- 
able or through environmental compulsion, but as 
the result of the beginnings of a social appetite. In 
early stages of such reactions, the movement together 
may come primarily because the collection of isopods 
or earthworms or starfishes are substitutes for miss- 
ing elements in the environment. 

Take, for example, the snake or brittle starfishes 
of the New England coast. These are rare now along 
Cape Cod, but before the wasting disease swept away 
the eel grass they were abundant in favorable locali- 
ties, but were rarely found close together. I have 
spent hours peering down through a glass-bottomed 
bucket here and there and round about in one of 
these localities, and have not seen more than one 

PLATE III. a. Brittle starfish aggregate readily 
when put into a bare vessel of sea watei . b shows con- 
ditions ten minutes after a was taken. (Photographs by 


at a time. And I have spent more hours wielding a 
sturdy garden rake in swathe after swathe through 
the short eel grass, very rarely pulling in more than 
one starfish at a haul. 

Yet when a few brittle starfishes are placed in a 
clean bucket of sea water they clump together like 
magic (Plate III). In bare laboratory aquaria they 
remain so clumped for weeks; in fact the aggrega- 
tions become more compact as time goes on as the 
animals bring back their extending arms and tuck 
them into the mass. If, however, the aquaria are 
dressed up by the introduction of eel grass so that 
conditions approach those found in nature, the ag- 
gregations disperse and the starfishes climb actively 
about over the blades of the eel grass, feeding on 
organisms and debris found on their surfaces. 

The idea that in clean laboratory dishes these star- 
fishes are substituting each other for the missing eel 
grass was obvious and easy to test. A kind of artifi- 
cial eel grass was made of glass rods twisted in 
various shapes so that they offered a supporting 
framework for climbing in much the same way as 
the true eel grass. So long as the rods remained the 
starfishes clambered about over the meshwork or 
hung motionless, usually isolated. If the rods were 
removed they again clustered together. 

As I have said elsewhere, (3) it is a far cry from 


such aggregations to the groupings of foreigners in 
a strange city that result in Little Italy, or the 
Mexican settlement, or a German quarter; and yet 
basically some of the factors involved are similar. 
Perhaps there is a closer connection between such 
aggregations in the wide expanse of a clean aqua- 
rium and the schooling tendency found among 
many fishes of the open sea; perhaps the same phe- 
nomenon accounts for the flocking tendency of 
many birds, as well as mammals on the equally 
monotonous grassy seas of temperate plains. 

A somewhat different expression of a positive 
social reaction is shown when animals that are 
usually more or less isolated come together and pass 
the night grouped as though they were engaged in 
a slumber party. This type of behavior has been 
repeatedly described for different insects, even for 
the wasps that remain separate to such an extent 
that they are called solitary wasps. In some forms of 
solitary wasps both males and females are found in 
the sleeping group. With solitary bees, such as we 
have near Chicago, the overnight aggregations are 
composed of males only. A study which was made 
of the sleeping habits of a Florida butterfly species 
indicates that these Heliconii (69) come together 
night after night in the same location, in part at 
least as a result of place-memory. The assemblages 


lack sexual significance. There is some protection in 
the fact that if one is disturbed the whole group may 
be warned. The presence of many butterflies would 
reinforce any species odor that might attract others 
of the same species, or repel possible predators. 

The crowded roosts to which certain birds return 
not only for one season but sometimes for years are 
widely known. Here again we are concerned with a 
positive social appetite which grows stronger with 
the approach of darkness; the details as to why and 
how it operates are not known. 

Animals which come together in intermittent 
groupings like these overnight aggregations are 
showing a social appetite which is none the less 
real because it is effective only at spaced intervals. 
In this it resembles other appetites such as those 
for food, water and sex relations. From such occa- 
sional or cyclic expressions of a social appetite it is 
a relatively short step to whole modes of life which 
are dominated by a drive for social relationships. 
As I have already said, in the insects alone this step 
has been taken some twenty-four distinct times and 
in widely separated divisions of that immense group. 

Normally the development of highly social life 
comes by way of an extension of sexual and family 
relations over greater portions of the life span. 
Here again all degrees of increased length of asso- 


elation can be shown, from the sexual forms that 
meet but once and for a brief moment to the ter- 
mite kings and queens that live together for years. 
Also all stages exist in the evolution of the associa- 
tion of parents with offspring, from the insects like 
the female walking-stick, which deposits eggs as she 
moves about and pays no more attention to them, 
to the ants and bees whose worker offspring spend 
their entire lives in the parental colony or some 
colony budding off from it. 

While the extension of family relations is very 
obviously one potent method by which social life is 
developed to a high level, there are other social 
groupings which also deserve consideration in con- 
nection with the problem as to the method of evo- 
lution of social life. Schools of fish arise, for exam- 
ple, under conditions in which there is no associa- 
tion with either parent after the eggs are laid. At 
times the eggs may be so scattered in the laying that 
the schools form from unrelated individuals. Here 
the schooling tendency seems to underlie rather 
than grow out of family life. The mixed flocks (22) 
of tropical birds which are composed of many spe- 
cies obviously did not grow directly from family 
gatherings, and the groups of stags of Scottish deer, 
probably the original stag parties, (38) appear to 
give evidence of a grouping tendency independent 


of intersexual or family relations. This subject will 
be discussed in more detail in the final chapter. 

The conclusion seems inescapable that the more 
closely-knit societies arose from some sort of simple 
aggregation, frequently, but not necessarily, solely 
of the sexual-familial pattern. Such an evolution 
could come about most readily with the existence 
of an underlying pervasive element of unconscious 
co-operation, or automatic tendency toward mutual 
aid among animals. 

In the simpler aggregations evidence for the pres- 
ence of such co-operation comes from the demon- 
stration of survival values for the group. These are 
more impressive the more constant they are found 
to be. If they exist throughout the year they are 
much more important as social forerunners than if 
present only during the mating season or at times 
of hibernation. 


■ Beginnings of Co-operation 

WITH this chapter I begin the presentation of the 
evidence for the assertion that there is a general prin- 
ciple of automatic co-operation which is one of the 
fundamental biological principles. The simplest ex- 
pression of this is often found in the beneficial ef- 
fects of numbers of animals present in a population. 
Laboratory work of the last two decades still shows 
that overcrowding is harmful, but it has also uncov- 
ered a no less real, though somewhat slighter, set of 
ill effects of undercrowding. 

To be sure, overcrowding always produces ill ef- 
fects, and these can always be demonstrated at some 
population density. On the other hand, the ill effects 
of undercrowding cannot always be shown, though 
frequently they can. In generalized curves the mat- 
ter may be summarized thus: Under certain condi- 
tions (g6) we find the curve running like the dia- 
gram in Figure 2 A, when height above base line 
gives the rate of the biological action being meas- 
ured, and distance to the right shows a steadily in- 



creasing population. Under these conditions only 
the ill effects of overcrowding are visible, and the 
optimum population is the lowest possible. This is 
the modern expression of what used to be called the 
struggle for existence. In the more poetic post-Dar- 
winian days this struggle was thought of as so in- 
tense and so personal that an improved fork in a 
bristle or a sharper claw or an oilier feather might 
turn the balance toward the favored animal. Now 
we find the struggle for existence mainly a matter 
of populations, measured in the long run only, and 
then by slight shifts in the ratio of births to deaths. 

A second type of phenomena is represented by a 
curve with a hump near the middle (97) as shown in 
Figure 2B. 

Again, height above the base line measures the 
speed of some essential biological process or proc- 
esses, such as longevity; distance to the right gives 
increasing population densities. The harmful effects 
of overcrowding, indicated by the long slope to the 
right, are still plainly evident, but there is also ap- 
parent a set of ill effects associated with undercrowd- 
ing which are shown by the downward slope to the 
left. Many have written pointedly about overcrowd- 
ing, and while there is still much to be learned in 
that field, it is in the recently demonstrated exist- 
ence of undercrowding, its mechanisms and its im- 



plications, that freshness lies. Without for one min- 
ute forgetting or minimizing the importance of the 
right-hand limb of the last curve, it is for the more 
romantic left-hand slope that I ask your attention. 

Fig. 2. A. Under some conditions the rate of bio- 
logical action which is being measured is greatest with 
the smallest population, and decreases as the numbers 
increase. B. Under other conditions there is a distinct 
decrease in the rate of the measured biological reaction 
with undercrowding (to the left) as well as overcrowding 
(to the right). 

Perhaps the simplest and most direct demonstra- 
tion of certain harmful effects of undercrowding 
comes from an experiment which I understand is 
carried on spontaneously among undergraduate men 
at certain universities and colleges of which X, or 
perhaps better, Y, is an example. A certain number 
of men gather together in a limited space under arti- 
ficial light and undertake to consume a more or less 
limited amount of stronger or weaker alcohol. If 


there are many men present in proportion to the 
amount of alcohol, relatively little or no harm will 
result from the experiment. If there are very few 
men and much alcohol there may be garage bills and 
other important repairs to be made. 

In one way or another similar tests have been car- 
ried out in the laboratory with a variety of poisons, 
and many kinds of animals. Again I choose from 
the mass of available evidence the results of a simple 
and clean-cut experiment to illustrate the same point 
with non-human animals. 

Everyone is acquainted with goldfish; they are 
hardy forms or else they would not be alive today 
in so many goldfish bowls. Colloidal silver in its 
commercial form of argyrol is also well known. Col- 
loidal silver, that is, the finely divided and dispersed 
suspension of metallic silver, is highly toxic to liv- 
ing things, including even the hardy goldfish. 

In the experiment in our laboratory (8) we ex- 
posed sets of ten goldfish in one liter of colloidal 
silver, and at the same time placed sets of ten simi- 
lar goldfish, one each, in a whole liter of the same 
strength of the same suspension. This was repeated 
until we had killed seven lots of ten goldfish and 
their seventy accompanying but isolated fellows. 
Then when the results were thrown together we had 
the simple table on page 54. 

t^- I 



Survival in minutes of goldfish in colloidal silver 


7X lo 70x1 
182 min. 507 min. 325 min. P < o.ooi 

Any biological experiment has a large number of 
so-called variables, that is, of factors that it is diffi- 
cult or impossible to bring under such complete 
control that we can be certain that the experiment 
will be exactly repeatable next time. Hence it is 
customary to make experiments if possible as paired 
experiments, in which one set of conditions (those 
of the group in this instance) will differ from an- 
other lot (those of the isolated goldfish) only by the 
one difference, in this case of grouping and isolation. 
Such results with these fish can then be analyzed 
by statistical methods to find the probability of get- 
ting like results merely "by chance." These methods 
are now so simple that even I can make the calcula- 
tions. They are as accepted a technique as is the 
paired experiment. 

With the goldfish there is less than one chance in 
a thousand of getting as great an average difference 
with the same number of trials. Technically we say 
that probability, or P, for short, is less than 0.0001. 


It means the same. Students of statistics have found 
that when P z= 0.05 or less, that is, when there are 
fewer than five chances in a hundred of such a thing 
happening as a result of random sampling or 
"chance," there is likely to be something significant 
in such results, the more so the smaller the fraction 
which P is said to equal. 

We make such tests of our experimental results 
continually, to find how we are getting on, and I 
shall give probabilities repeatedly. In doing so it 
must be remembered that these test the data, not 
the theory— and that the data may vary significantly 
for unknown reasons, even when we think we are 
in full control of the situation; and that because 
there is only one chance in one hundred, or ten 
thousand, or a million that a thing may happen by 
"chance" does not mean that it will never happen 
through what we call an accident; merely that the 
chances of its happening so, our evidence being what 
it is, are on the order of one in one hundred, or ten 
thousand, or a million. 

I will digress even further into the realm of coinci- 
dence. A Negro friend of mine spent a summer in 
Europe and while in Paris visited the art galleries 
of the Louvre. While there he saw a Negro woman 
busy looking at pictures and on coming closer dis- 
covered that she was his own aunt. Neither had any 


idea that the other was in Europe. With no pre- 
arrangement, what is the probability that an Ameri- 
can Negro from Chicago will meet his aunt in the 
Louvre? Yet it did happen this once without in any 
way shaking the probability principle. 

Perhaps the digression is not so great as might ap- 
pear at first glance, for we need a slight common 
understanding of the practical working of statistical 
probability; all of modern science, the more as well 
as the less exact, is built on it. 

To get back to our goldfish: those in the groups 
of ten lived decidedly longer than their fellows ex- 
posed singly to the same amount of the same poison; 
and significantly so. But why? Others had made that 
experiment w^ith smaller animals, and had decided 
that the group gave off a mutually protective secre- 
tion which would protect that particular species and 
none other. One reason that we were working with 
goldfish was because they are large enough so that 
we could use approved methods of chemical analysis 
in finding where the silver went. The balance sheet 
from such tests showed that we could account for 
all the silver present. With the suspensions which 
had held ten fish the silver was almost all precipi- 
tated, while in the beakers that had held but one fish 
almost all the silver was still suspended. 

When exposed to the toxic colloidal silver the 


grouped fish shared between them a dose easily fatal 
for any one of them; the slime they secreted changed 
much of the silver into a less toxic form. In the ex- 
periment as set up the suspension was somewhat too 
strong for any to survive; with a weaker suspension 
some or all of the grouped animals would have lived; 
as it was, the group gained for its members a longer 
life. In nature, they could have had that many more 
minutes for rain to have diluted the water or some 
other disturbance to have cleared up the poison and 
given the fish a chance for complete recovery. 

With other poisons, other mechanisms become 
effective in supplying group protection. Grouped 
Daphnia, (50) the active water fleas known to all 
amateur fish culturists, survive longer in over-alka- 
line solutions than daphnids isolated into the same 
volume. The reason here is simple. The grouped 
animals give off more carbon dioxide, and this neu- 
tralizes the alkali. Long before the isolated individual 
can accomplish this, it is dead; in the group those 
on the outside may succumb, though if the num- 
ber present is large enough even they may be able 
to live until the environment is brought under tem- 
porary control. 

Frequently the protective mechanism is much 
more complex. With many aquatic animals, other 
things being equal, isolated animals consume more 


oxygen than if two or more share the same amount 
of liquid. By one device or another, grouping fre- 
quently decreases the rate of respiration. Several of 
these devices are known to us. Professor Child 
showed many years ago (31) that when animals are 
exposed to a strongly toxic material, those with the 
higher rate of respiration, though otherwise similar, 
die first. This has been applied to group biology by 
direct tests, and it has been shown that the group, 
by decreasing the rate of oxygen consumption of its 
members, makes them more resistant to the action 
of relatively strong concentrations of toxic materials. 

Perhaps I have said enough to show that under 
a variety of conditions groups of animals may be 
able to live when isolated individuals would be 
killed or at least more severely injured by unaccus- 
tomed toxic, chemical elements, strange to their nor- 
mal environment. 

Will the same relationship hold in the presence 
of changes in physical conditions? There is a con- 
siderable and growing lot of evidence that massed 
animals, even those that can be called cold-blooded, 
are harder to kill by temperature changes than are 
similar forms when isolated. (51, 126) This interests 
us because massing of such animals at the onset of 
hibernation was recognized as one of the early ex- 


ceptions to the rule, now outgrown, that crowding 
is always harmful. 

The exploration of temperature relations is a 
time-honored field. I prefer to take up a newer 
though related area, that of the effects of ultra-violet 
radiation, in which I shall present some evidence 
so recently collected that it has never been reported 
extensively before. A year ago Miss Janet Wilder 
and I began exposing the common planarian worm 
of this region to ultra-violet radiation, to find 
whether there was any group protection from the 
well-described lethal effect of ultra-violet light on 
these worms. (12) 

In lots of twenty, worms of similar size and the 
same history were placed together in a petri dish and 
exposed to the action of the ultra-violet light long 
enough so that they would disintegrate within the 
next twelve hours. Half of them, that is, ten worms, 
were then placed together in five cubic centimeters 
of water and each of the other ten was put into five 
cubic centimeters of similar water. Grouped and iso- 
lated worms were treated alike in every way, except 
that after irradiation together, half were grouped and 
half were isolated. 

For one purpose or another we have repeated this 
simple experiment a great many times with a variety 
of waters, and with experimental conditions ade- 


quately controlled. Some of the things we have found 
out are: 

If the worms are crowded under the ultra-violet 
lamp so that they shade each other, the shaded ones 












IH 168' 
tI ItI 
ooooojjj HH^oooooJUJ I 


140 ^M 
P I 




Fig. 3. Planarian worms which have been exposed to 
ultra-violet radiation disintegrate more rapidly if isolated 
than if grouped. 

are definitely protected. When such crowding is 
eliminated and by constant watching and stirring, 
if needed, during exposure, the worms are kept ap- 
proximately equally spaced, even then the grouped 
worms survive longer than if isolated. Some of the 
relationships are shown in Figure 3. 

Each block represents the survival time of several 
series of worms. The figures at the top of the block 
give the average length of survival in minutes. The 
blocks are constructed so that the worms surviving 


longer, which in each case are the grouped worms, 
are given as lOO per cent, regardless of the time 
taken; while the isolated worms, which had been 
irradiated in the same dishes as their accompanying 
groups, survived on an average of 78 per cent and 
77 per cent respectively in the two tests with well 
water, and only 61 per cent in the test in dis- 
tilled water. The numbers between the blocks show 
the number of worms averaged for each block; that 
is, the number of pairs of worms for which results 
are summarized. The statistical significance given 
in terms of 'T" is very high in each case. 

The number present during exposure is impor- 
tant, as well as the number present during the time 
when it is being determined how long the animals 
will survive. Such data are summarized in Figure 4, 
which is built exactly on the same principle as that 
preceding. Worms radiated when crowded (left-hand 
block), and then tested when isolated, survived 517 
minutes, while accompanying worms which had been 
radiated singly as well as tested when isolated, lived 
only 24 per cent as long. Those radiated in a group 
and tested singly (middle block) lived 55 per cent as 
long as those which had been radiated in a crowd 
and then were isolated to observe the effects of radi- 
ation. It will be remembered that these crowded 
worms actually shaded each other and so gave 


physical protection from the ill effects of ultra-violet 
light. Finally (on the extreme right) is diagramed 
the fact that worms radiated and tested singly lived 
only 62 per cent as long as those radiated in a group 




Singly singly Singly singly singly singly 

517' 517' 107' 


Fig. 4. Planarian worms survive exposure to ultra- 
violet radiation better if much crowded while being 
radiated, or even partially crowded, even though all are 
isolated after a few minutes of irradiation. 

of 20 per 20 cubic centimeters and also tested singly. 
Again the figures give the number of pairs tested 
and under "P" the statistical probability, which 
shows that all these must be taken seriously even 
though there is decreasing significance as the per- 
centage of difference of average survival time de- 

In the two cases just outlined mass protection has 
been demonstrated, first against the presence of toxic 


materials, and second against the ill effects of expo- 
sure to lethal ultra-violet rays. To complete the pic- 
ture I have now to describe the results of exposing 
animals to harmful conditions in which the difficulty 
is caused by the absence of elements normally pres- 
ent in their natural environment. The experiment 
has been made on aquatic animals in a number of 
ways, for example, by putting fresh-water animals 
into distilled water; but it is easier to demonstrate 
when marine animals are placed in fresh water. 

Again I select one experimental case from several 
available. Near Woods Hole, on Cape Cod, a small 
flatworm Procerodes (Figure 5) lives in certain re- 
stricted areas in large numbers. They are most abun- 
dant along a stony stretch at about the low tidemark 
or a little beyond it. (5) There, if one finds the proper 
location, one may take from ten to fifty flatworms 
from the lower surface of a single stone. Usually 
they are more or less clumped together. They are 
not easy to see since each is only a few millimeters 
long and all are of a dull gray color. Once seen, 
they are hard to detach, for the posterior end has a 
muscular sucker, by means of which the animal can 
cling pretty securely even to smooth stones. When 
these worms are put into fresh water, pond water for 
example, they swell greatly and soon begin to dis- 




If these flatworms are washed thoroughly to re- 
move sea water from their surfaces, and then placed 
in fresh water, a certain proportion of the grouped 

Fig. 5. The small marine flatworm Procerodes. 

animals survive decidedly longer than isolated worms. 
The first worms to die in the group do so almost 
as soon as the first isolated worms. As the dead worm 
disintegrates it changes the surrounding water; we 
say it conditions it; and as a result of this condition- 
ing the remaining worms of the group have a bet- 
ter chance of life. 


For more careful experimentation, a sort of worm 
soup was prepared by killing a number of well- 
washed worms and allowing them to remain in the 
water in which they had died and so condition it. 
Freshly collected Procerodes lived longer in such 
conditioned water than their fellows which were 
isolated into uncontaminated, clean pond water. The 
difference between the two waters was only that 
caused by the fact that in one the worms had died 
and disintegrated, while the other was clean. This 
difference in survival persisted even when, to make 
the test more revealing, the total amount of salt in 
the two waters was made identical by adding some 
dilute sea water to the clean pond water. Results 
from these experiments are shown in Figure 6. In 
this chart, distance above the base line gives the 
percentage of survival, and distance to the right 
shows time of exposure. It will be noted that the 
worms lived decidedly longer in the conditioned 
water than they did in dilute sea water of the same 
strength of salts. 

The mechanism of this superficially mysterious 
group protection is now known. (86) The dead and 
disintegrating worms, or more slowly, the living 
worms, give off calcium into the surrounding water, 
and calcium has a protective action for marine ani- 
mals placed in fresh water or for fresh-water animals 



put into distilled water, a protective action which is 
out of all proportion to its effect in increasing the 
osmotic pressure of the water. We can demonstrate 
that this is in fact the mechanism of such group 

1 Conditioned water 

Fig. 6. Procerodes die more rapidly if transferred to 
pure fresh water than in dilute sea water, but live 
longer if placed in fresh water in which other Procerodes 
worms have died, even though the total amount of salt 
is the same as in the dilute sea water. 

protection. For example, we can analyze the water 
which worms have conditioned, find the amount of 
calcium that has been added, and by adding that 
amount directly get the same results that we do from 
the conditioned water. 

This explanation is not yet complete— no scientific 
explanation ever is— but we have demonstrated that 
what was for a time a very mysterious group pro- 


tection is in fact in this case an expression of calcium 
physiology. The further developments on the sub- 
ject await exact information concerning the details 
of the physiological effects of calcium. 

It is probably of more direct human interest to 






Fig. 7. Bacteria frequently do not grow if inoculated 
in small numbers; here different numbers of Bacillus 
coli were inoculated into a medium containing gentian 

know that under many conditions bacteria will not 
grow if only a few are inoculated into an animal, 
man for example; while with a larger inoculation 
they may grow abundantly. (33) Gentian violet is a 
poison for many bacteria and in regular medical use 
for that purpose. In one well-studied case (Figure 7) 
bacteria belonging to the species Bacillus coli failed 
to grow on agar containing gentian violet, if singly 
inoculated on it; only when thirty or more bacteria 


were inoculated did steady and regular growth oc- 
cur. With the goldfish spoken of earlier, the mass 
protection was largely or wholly inoperative when 
the group of ten was exposed to ten times the amount 
of toxic colloidal silver to which a single fish was 
exposed. With these bacteria, however, such quanti- 
tative limitations did not hold; thirty organisms 
were found to fix at least two hundred times the 
amount of poison normally neutralized by an iso- 
lated bacterium. This difference between the change 
which thirty bacteria can effect together as compared 
with what they can accomplish if isolated has been 
called an expression of the communal activity of 
bacteria. There is a fairly large and growing litera- 
ture on this subject which indicates that when only 
one or a few bacteria, even if strongly pathogenic, 
gain access to the human body, they are likely to 
be killed by various devices which aid in resisting 
infection. It is fortunate for their victims that bac- 
terial infections normally tend not to take unless 
the inoculum is somewhat sizable or unless a smaller 
dose is frequently repeated. 

Mass protection is known to occur among sper- 
matozoa. Many animals, especially those that live in 
the ocean, shed their eggs and spermatozoa into the 
sea water, and fertilization takes place in that me- 
dium. Dilute suspensions of such spermatozoa lose 


their ability to fertilize eggs much sooner than if 
they are present in greater concentration. It is rou- 
tine laboratory practice in experimenting with such 
animals as the common sea-urchin, Arbacia, to keep 
sperm in a cool place, densely massed outside the 
body, for hours. Small drops can be withdrawn as 
needed for experimentation, greatly diluted and 
used almost immediately to fertilize eggs. When such 
dilute suspensions have long since lost their fer- 
tilizing power the sperm in the original dense mass 
are still potentially as active as ever. 

So far we have been considering mass effects, the 
survival value of which, if any, was shown by in- 
creased length of life, often under adverse circum- 
stances. Under many different conditions and for a 
variety of organisms, the presence of numbers of 
forms relatively near each other confers protection 
on a part of those grouped together or even on all 

It is possible to go a step farther and demonstrate 
a more actively positive effect of numbers of or- 
ganisms upon each other when they are collected to- 
gether. Again I select a fresh case for close scrutiny; 
that of crowding upon the rate of development in 
sea-urchin eggs. 

Arbacia, mentioned above, is the common sea- 
urchin of coastal waters south of Cape Cod (Fig- 


ure 8). It has been much used in studies of various 
aspects of development, particularly by the biologists 
who gather each summer in the research laboratories 
at Woods Hole, Massachusetts. There are several 
reasons for its popularity. These urchins are abun- 

FiG. 8. Arbacia, the common sea-urchin of southern 
New England, shown from the upper surface. 

dant in near-by waters and are readily mopped up by 
the tubful. They can be kept in good condition for 
some days in the float cages, and eggs and sperm 
are readily procured as needed. Also the breeding 
season of Arbacia extends through July and August, 
which are favored months for research at the seaside. 
For years biologists at Woods Hole have studied 
the embryology and physiology of developing sea- 
urchin eggs. They have built up a painstaking, 
almost a ritualistic, technique for handling glassware, 


towels and instruments. The procedures require as 
rigid cleanliness as a surgical operation. Conse- 
quently it was not surprising when I first took up 
their study a few years ago, to have one of my frank- 
est friends among the long-time workers on the de- 
velopment of Arbacia, voice what was apparently a 
common feeling among them. He asked pointedly 
if I thought I could come into that well-worked field 
and without long training find something they had 
overlooked. Such frank skepticism was refreshingly 
stimulating and added to the normal zest of bio- 
logical prospecting. 

The shed eggs of Arhacia are about the size of 
pin points and are just visible to the naked eye. The 
spermatozoa are tiny things; the individual sperm 
are invisible without a microscope although readily 
seen when massed in large numbers. When a few 
drops of dilute sperm suspension are added to well- 
washed eggs, one spermatozoan unites with one q^^. 

After some fifty minutes at usual temperatures, the 
egg divides into two cells. We call this the first 
cleavage. Thirty or forty minutes later a second 
cleavage takes place and thereafter cleavages occur 
rapidly. Within a day, if all goes well, such an egg 
will have developed into a freely swimming larva. 
Other things being equal, (lo) the time after fer- 
tilization to first, second and third cleavage is speeded 



up for the crowded eggs. Typical results and some 
of the methods are shown in Figure 9. With appro- 

4- mm.' 


% cleai/ed 

% cieaued. 

58.25 60.25 

85.83 90.25 

99 /GO 

Fig. 9. Eggs of the sea-urchin, Arhacia, cleave more 
rapidly in dense populations than if only a few are 
present. Figures below the diagrams, unless otherwise 
indicated, give time in minutes. 

priate experimental precautions, some eighteen hun- 
dred eggs were introduced into a tiny drop of sea 
water. Near by on the same slide forty similar eggs 
were placed in a similar drop and the two were 
connected by a narrow strait as shown in the figure. 


A few eggs from the larger mass spilled over into 
this strait. The whole slide was placed in a moist 
chamber to avoid drying, and examined from time 
to time. In a trifle over fifty-five minutes half the 
eggs in the densest drop had passed first cleavage. A 
half-minute later, 50 per cent of those in the strait 
were cleaved, and twenty seconds later half of the 
more isolated ones had divided. The time to 50 per 
cent second cleavage ranged between eighty-four 
minutes for the crowded eggs and over eighty-six 
and a half minutes for the isolated ones. 

This was repeated with four thousand eggs or 
thereabouts in the denser population, almost six 
hundred of which spilled through and formed a flat 
apron over the bottom of the second drop, in which 
there were thirteen other eggs scattered singly about 
the relatively unoccupied space. Under these condi- 
tions the time to 50 per cent first cleavage was ap- 
proximately fifty-two, fifty-eight and sixty minutes 
respectively, and the difference at the middle of the 
second cleavage was even greater. 

In association with Dr. Gertrude Evans, who is a 
good, skeptical research worker, this experiment was 
repeated in many different ways; and there remains 
in my mind no doubt but that under a variety of 
conditions the denser clusters of these Arhacia eggs 


cleave more rapidly than associated but isolated 

Under the conditions tested, the stimulating effect 
of crowding could be detected when sixty-five or 
more eggs were present in the more crowded drop 
and twenty-four or fewer eggs made up the accom- 
panying sparse population. 

Within twenty-four hours, under favorable condi- 
tions, one finds one's cultures full of free-swimming 
larvae with characteristic arms which are known as 
plutei. When all our available data collected the 
first day after fertilization are compared there is 
again no doubt but that the more crowded cultures 
usually develop more rapidly than accompanying but 
sparser populations. However, it must be recorded 
that throughout the whole series there were occa- 
sional isolated eggs that developed as rapidly as the 
best of the accompanying denser populations. Such 
eggs and embryos were exceptional in our experi- 
ence; the fact that they exist indicates clearly that 
under the conditions of our experiments crowding, 
while usually stimulating, was not absolutely neces- 
sary for rapid cleavage and early growth. 

In this connection it is interesting to note that 
others have prepared an extract from sea-urchin eggs 
and larvae which is growth-promoting, (91) and one 
which is growth-inhibiting. As has also been found 


with goldfish, the growth-accelerating principle seems 
to be associated with the protein fraction of the ex- 
tract. When the whole extract is used, it is said to 
be growth-inhibiting and to produce the same re- 
sults as overcrowding. The point I have made is 
that with the sea-urchin eggs, under the conditions 
of our experiments, there is also an ill effect of un- 
dercrowding, and that there is an optimum popula- 
tion size for speedy development which is neither 
too crowded nor too scattered. 

Much similar work has been done with the ef- 
fects of numbers on the rate of multiplication with 
various protozoans. Again I shall have to select re- 
sults from the mass of available evidence. The late 
T. Brailsford Robertson (107) of Australia an- 
nounced back in 1921 that when two protozoans of 
a certain species were placed together, the rate of 
division was considerably more than double that 
which resulted with only one present. It should be 
noted that during the time of these experiments and 
in all these protozoa which we are considering re- 
production was entirely asexual, by self-division of 
the original animal. I subjected the data in Robert- 
son's original paper to statistical analysis and found 
that there were only thirteen chances in a thousand 
of getting as great a difference by random sampling. 
Such results must be taken seriously (Figure 10). 


They were. And the period after 1921 was en- 
livened for some of us by denials from one first- 
class laboratory after another that there was anything 
significant in Robertson's data. Robertson himself 


24 HOURS 20.5 92.4 

RATIO 1 2.2 

I 44 


P = 0.0128 

Fig. 10. Robertson found that when two protozoans 
were placed together each yielded over twice as many as 
when the same number of similar protozoans were iso- 

rechecked and confirmed his results, though his ex- 
planations of them tended to vary. For the moment 
we are not concerned with the explanations; but 
what are the facts? The first extensive corroboration 
from outside Robertson's own laboratory came from 
the work of Dr. Petersen at Chicago. When she cul- 
tured the common Paramecium in small volumes of 
liquid, she obtained the same results as had Rob- 
ertson's critics, but when she used relatively larger 
volumes of the same culture medium, a cubic cen- 


timeter more or less, she got an increase in division 
rate with the presence of a second individual, as 
Robertson had found it in the Australian form he 
had studied. 

Still the critics were not convinced. Accordingly 
Dr. Johnson, now of Stanford University, repeated 
this whole study using a different protozoan, one of 
the Oxytricha. (68) When sister cells from pure-line 
cultures were used there was no difference at the 
end of the first day, whether the Oxytricha were in- 
troduced singly or in pairs into one or two drops of 
good medium. Later, the cultures started with one 
organism always were ahead. With larger volumes, 
two organisms showed a higher rate of reproduction 
per original animal at the end of the first day than 
if started with a single protozoan. 

Again for larger volumes Robertson's results were 
confirmed, and those of his critics for smaller vol- 
umes. But Johnson had only started. He knew from 
the work of others that if a protozoan is washed 
through several baths of sterile water the associated 
bacteria are rinsed off. Then if the washed protozoan 
is put into a weak solution of the proper salts, into 
which has been introduced known numbers of the 
bacteria on which they normally feed, the problem 
can be studied with a controlled food supply, both 
as to kind and amount. 


This he proceeded to do. He found a common 
bacterium on which his sterile Oxytricha would grow 







3.5 9.0 11.4 5.4 3.0 

Fig. 11. The ciliate protozoan Oxytricha reproduces 
more rapidly with a certain limited number of bacteria 
present than with either more or fewer. (From Johnson.) 

and reproduce faster than in the ordinary medium. 
He made standard suspensions of these bacteria in 
sterile salt solution, at what we may call an X con- 
centration. The bacteria could reproduce little, if at 
all, in the salt medium, so that he knew how much 


and what kind of fodder he was feeding his washed 

The resuhs of varying the amount of food are 



seeoiNG t 2 

8.0 tO.2 tO.6 10.4 

Fig. 12. In the denser suspensions of bacteria the 
protozoans divide more rapidly when cultures are inocu- 
lated with two protozoans than if started with a single 
individual. (From Johnson.) 

shown in Figure ii. With X concentration, in 
twenty-four hours one animal produced about eleven 
progeny. With 2X concentration, isolated sister cells 
produced nine, and with a 4X concentration other 
isolated sister cells produced but three and a half. 
The rate of reproduction also decreased when less 
than X bacteria were present. 


Now he was ready for the grand Robertson test, 
except that by this time nearly all the factors were 
controlled. The results are shown in the following 
figure (Figure 12). With X concentration it made 
no difference whether he started his small cultures 
with one or with two sterile animals. With 2X con- 
centration, the cultures started with two individuals 
did as well as in X concentration, but those which 
were started with only one individual lagged defi- 
nitely, producing only 80 per cent as many animals 
in twenty-four hours. With 4X concentration even 
the culture started with two Oxytricha was slowed 
down, but not so much as that started with only 
one. He had shown that in the presence of an ex- 
cess number of bacteria, cultures seeded with more 
than one bacterium-eating protozoan thrive better 
than if but one is introduced. Not content with this 
Johnson took another species and tried it all over 
again with the same results. 

From all this careful work we judge that the facts 
on this particular aspect of the effects of numbers 
present on the rate of asexual reproduction seem 
now to be straight; but what about their expla- 
nation? This, as it turns out, also interests us. 
Robertson advanced the following hypothesis to 
explain the results which he had observed. Dur- 
ing division each nucleus retains as much as pos- 


sible of an essential, growth-producing substance 
with which it was provided, and adds to it dur- 
ing the course of growth between divisions. At 
each division, however, this substance is necessarily 
shared with the surrounding medium in a propor- 
tion that is determined by its relative solubility in 
the culture water, and by its affinity for chemical 
substances within the nucleus. The mutual speeding 
of division by neighboring cells is due to each cell's 
losing less of this necessary substance because of the 
presence of the other. The more of this growth-pro- 
moting substance there was in the cell, Robertson 
thought, the faster would be the division rate; so 
that any circumstance which would conserve the 
limited supply would tend to speed up processes 
leading to cell division. 

Stripped to essentials this hypothesis says that as 
a result of the presence of a second organism both 
lose less of an unknown something which is essen- 
tial for division than would happen if but one were 
present. Returning to the problem after the criti- 
cisms of half a dozen years, Robertson affirmed that 
all the data and conclusions on the subject that had 
been issued from his laboratory remained valid save 
that they might apply to the ^ associated food or- 
ganisms and not to the protozoans themselves. 

Johnson has paid considerable attention to this 


problem, and has concluded that the results which 
he has observed can be explained as due to bacterial 
crowding; that the larger number of protozoans in- 
troduced into dense cultures thrive best because they 
are able to reduce the bacteria to density optimal 
to the protozoa faster than their isolated sister cells 
can; and therefore they show a higher rate of re- 

This does not seem to be the whole story; for from 
points as distant as Baltimore (79) and Jerusalem, 
(101) I have reports from trustworthy men that with 
still simpler protozoans they are getting results which 
suggest that some modification of Robertson's hy- 
pothesis may be correct after all. These organisms 
stimulate each other to more rapid growth merely 
by their presence in the same small space. 

With fine courtesy, Professor Mast of Johns Hop- 
kins has placed a report of his experiments in my 
hands in advance of publication and has permitted 
me to summarize his results. He finds that popula- 
tions of a flagellate protozoan grow more rapidly in 
a sterile medium of relatively simple salts when 
larger numbers are introduced than if the cultures 
are started with only a few organisms. 

I must not put too much stress on these reports, 
pending the appearance of yet more data, but I 
should expect to find here, as elsewhere, that com- 


plicated problems such as these that deal with the 
rate of population growth are controlled by more 
than one mechanism. 

The suggestions from the simpler protozoans, 
taken together with other aspects of the mass physi- 
ology of protozoa which have been only partially 
reviewed here, and with the acceleration of devel- 
opment demonstrated for sea-urchin eggs, encourage 
me to renew a suggestion made some years ago, (3) 
which has, so far as I am aware, been overlooked 
to date. 

Let us go back to consider the case of external 
fertilization among aquatic animals. When sperma- 
tozoa and eggs are shed into sea water by sea- 
urchins or other marine animals, their length of 
life is distinctly limited. If a sperm fails to contact 
an egg during the fertilizable period, death results 
probably from starvation for the spermatozoa, per- 
haps from suffocation for the egg. This means that 
the animals of the two sexes must be fairly close 
together if there is to be a union of the shed sexual 
products. The most vigorous sperm of the sea- 
urchin Arhacia can travel in still water about 
thirty centimeters, that is, about one foot and two 
inches. (55) Spermatozoa of these animals diluted 
a few thousands of times can survive from three to 
twelve hours; the majority succumb by seven hours. 


If a current catches it, such sperm can travel many 
times thirty centimeters, but even in sea water the 
sexes must be relatively aggregated if fertilization 
is to be successful. In fresh water, the life of shed 
gametes is much shorter. After ten minutes, eggs of 
the pike lose the power to be fertilized, (102) and 
the longevity of sperm of certain fresh-water fishes 
is said to be less than a minute, so that in fresh 
water the aggregation is even more essential. With 
animals that require internal impregnation the 
necessity for close co-operation between at least 
two individuals is obvious. Such considerations must 
be fundamental for the long-recognized breeding 
aggregations of animals, especially of those that shed 
eggs and sperm into surrounding water. 

Mass relationships may be even more important 
sexually, and here I come to the new suggestion: 
perhaps they had a hand in shaping sex itself. Pre- 
sumably sexual evolution started, as it does today in 
plants, with a time when all gametes of any one spe- 
cies were similar. Under these conditions a first step 
toward the union of two reproductive elements 
could be supplied by the greater well-being fos- 
tered by the presence of more than one gamete 
within a limited area, as even the simpler proto- 
zoans are stimulated to asexual division today by the 
near-by presence of another of the same species. In 



the survival value existing for separate living cells 
before actual sexual union took place we can find a 
logical beginning for the action of selection, which 
would in turn, with present known values, result in 
the establishment of the sexual phenomena as they 
appear today. These fields have not been sufficiently 
explored to allow for more than this flash of imagina- 
tion, which future researches may verify or discard. 

At this point it would be well to pause and look 
back over the road we have traveled thus far. The 
charts, (7) shown as Figures 13 A and B, show that 
most of our evidence has come from fairly well down 
among the simpler forms of life. I have called atten- 
tion to mass protection of one sort or another among 
bacteria, planarian worms, goldfish and the simpler 
crustaceans. Actually there are in scientific literature 
good cases of mass protection for almost all the ani- 
mals shown in these charts; and where exact informa- 
tion is lacking, as for example among the rotifers, 
this is a result only of lack of interest in conducting 
experiments on this point with these animals. I have 
little doubt that we could, overnight, demonstrate 
mass protection from colloidal silver for rotifers; but 
we have more interesting work to do. 

I have also shown active acceleration of fundamen- 
tal biological processes as a result of numbers present 
for sea-urchin eggs and larvae, and for various pro- 

Bi(rdM M&mmalls 

Amphil^ani^ / 


Ancesrral plants 



Ancestral animal-plants — "^ 
Primitive protoplasm 
Fig. 13. A recent suggestion concerning the ancestral 
relations within the animal kingdom. The circles in A 






Ancestral plants 

Ancestral animal-plants — 

Primitive Protoplasm 

and B allow cross-identification. (From Allee in The 
World and Man.) 


tozoans. These have been given in some detail, which 
has not left time for similar demonstrations among 
regenerating cells of sponges; nor have I time to tell 
how hydra have been saved from depression periods 
by the use of self-conditioned water. I have men- 
tioned but not elaborated the fact that grouped ani- 
mals frequently have different rates of respiration as 
compared with their isolated fellows. This has been 
recorded widely in the animal kingdom, notably 
among planarians, certain lower crustaceans, some 
starfish, fishes and lizards, and for some, at least, asso- 
ciated survival values have been demonstrated. To 
this extent, then, I have given the crucial evidence 
I promised earlier that a sort of unconscious co- 
operation or automatic mutualism extends far down 
among the simpler plants and animals. 

These charts should illustrate one other point. The 
insects stand at the apex of one long line of evolu- 
tion; mammals and birds are at the peak of another 
line of evolution; the two have been distinct for a 
very long time. This view of evolution indicates that 
the ancestral tree of animals is not like that of a pine 
tree with man at the very top and insects and all the 
other animals arranged as side shoots from one main 
stem. Rather, there are at least two main branches 
which start low, as in a well-pruned peach tree. Both 
rise to approximately equal heights, indicating cor- 


rectly that in their way the insects are as specialized 
as the birds or mammals. Since both insects and 
mammals have developed closely-knit social groups, 
this is further evidence that there is a widely dis- 
tributed potentiality of social life. We shall return 
to this subject later. 


Aggregations of Higher Animals 

A GREAT deal of skepticism is necessary in science, 
if progress is to be even relatively steady and sound. 
Not only must the scientist be skeptical of advance 
reports of new results until he has seen the support- 
ing evidence, no matter how stimulating the thesis 
and how well it would explain material already 
gathered; but in fields which lie near his own re- 
searches it is necessary if possible to bring the prob- 
lem into his own laboratory and there examine the 
validity of the evidence itself. This repeating of ex- 
periments in order to check the first observer is some- 
times also a testing of scientific courtesy, but every 
real scientist must be prepared to submit to it with 
the best grace possible. 

It is demanded also that from time to time one 
should be skeptical of views long held, and of the 
evidence on which they were built up, particularly 
of the inclusiveness of the conclusions that have been 
drawn. Without my own fair share of this skepticism 
I should never have been drawn into what I knew 



from the beginning would be a long and laborious 
series of experiments concerning the effects of num- 
bers present upon growth. 

As long ago as the eighteen-fifties Jabez Hogg, 
(62) an Englishman, found by experimenting that 
crowding decreased the rate of growth of snails and 
produced stunted adults. From that day to this there 
has been almost no break in the reported evidence 
that overcrowding reduces growth; the number of 
reports that crowding in any degree increases growth 
are relatively few. 

This phenomenon has, however, been observed by 
enough workers using animals widely distributed 
through the animal kingdom to show that the retard- 
ing effect of undercrowding on growth is real. Before 
considering the implications of this statement let me 
review briefly some of the evidence. (3) Here as else- 
where I shall make no attempt to catalogue all the 
available evidence; the list would be impressively 
long but tedious. 

It is relatively easy to show that mixed populations 
of many animals grow faster than if the same number 
of some one species are cultured together. The com- 
mon experience of aquarium enthusiasts that the 
presence of the snails in aquaria increases the rate of 
growth and well-being of their fishes is a case in 
point. Their rule-of-thumb experience has been fully 


verified by careful laboratory experiments. A more 
crucial test involves individuals of the same species: 
all snails, let us say, or all goldfish. Is there some 
optimum size of the population at which individuals 
grow most rapidly? 

For years I have been studying different aspects of 
this problem with the aid of a succession of com- 
petent, critical research assistants and associates. The 
names of these young scientists are interesting and, 
I think, important. They include Drs. Bowen, Welty, 
Shaw, Oesting and Evans, and Messrs. Livengood, 
Hoskins, and Finkel, all of whom have independ- 
ently obtained the basic results I am about to de- 
scribe. (13, 14, 76) 

We have used goldfish for our experimental ani- 
mals, because these are inexpensive, easy to obtain, 
hardy under laboratory conditions, and able to stand 
daily handling. 

In order to have a consistently constant water we 
make up a synthetic pond water by dissolving in good 
distilled water salts of high chemical purity. Into 
such water goldfish about three inches long are 
placed in sufficient number so that they will give a 
conditioning coefficient of about twenty-five. Let me 
explain: this coefficient is obtained by multiplying 
the number of fish by their average length in milli- 
meters and dividing by the number of liters of water 


in the containing vessel. Living in this water the fish 
condition it by giving off organic matter and carbon 
dioxide. They are left in the water for twenty-one 
hours or so, while a similar amount of the same water 
stands near by under exactly similar conditions ex- 
cept for the absence of fish. 

At the end of this time the clean control water is 
siphoned into a number of clean jars, and a small 
measured goldfish is placed in each. At the same time 
the conditioned water is siphoned, either with or 
without removing particles (that is, of excrement, 
etc.) that may be present, into similarly clean jars. 
A set of small measured goldfish, like those used in 
the control jars, are transferred into the conditioned 
water. These small "assay" fish have been feeding 
for about two hours before being transferred; the 
larger conditioning fish are allowed to feed for a 
somewhat longer time before being washed carefully 
to remove food residues and replaced in another lot 
of water to condition that. 

Meantime the jars, 120 of them, are all washed 
carefully; and after this is done the experimenter has 
nothing more to do until the next day, except to put 
the laboratory in order, keep the temperamental 
steam distilling apparatus running, test the water 
chemically in several ways, keep his records in order, 


and otherwise see that nothing untoward happens to 
make him or anyone else question the results. 

After some twenty, twenty-five or thirty days of 
such care, in which Sundays are included, again each 
fish is photographed to scale, as they were also photo- 
graphed at the beginning of the experiment; the 
photographs are measured and the relative growth 
determined for the fish that have daily been placed 
into perfectly clean synthetic pond water, as com- 
pared with those which daily have been put into 
conditioned water, that is, into the water in which 
other goldfish have lived for a day. 

During the course of an analysis of this problem 
we have performed this simple basic experiment 
many times. The first forty-two such tests involving 
886 fish gave on the average about two units more 
growth for the fish in the conditioned, slightly con- 
taminated water, than for those in clean water (Fig- 
ure 14). These results have a statistical probability 
(P) of about one chance in a hundred million of 
being duplicated by random sampling. Hence we 
have demonstrated that under the conditions of our 
experiments the goldfish grow better in water in 
which other similar goldfish have lived than they do 
when they are daily transferred to perfectly clean 

The problem that has been occupying us for some 



time is why this is so. What are the factors involved 

that make this slightly contaminated water a better 

medium for young goldfish than a clean medium? 

We have said that the conditioning fish are fed 


«0. 290 274 


GROWTH 1.8 -0 2 

no. 180 142 



GROWTH 1.65 1.00 

MO. 210 120 




GROWTH 2.28 t.ia 

ma t6l 114 


GROWTH 1.92 

Ha 220 217 


GROWTH 2.59 2.20 

Fig. 14. Goldfish grow more rapidly if placed in vari- 
ous kinds of slightly contaminated (conditioned) water. 
The numbers above the columns show the number of 
fish tested. The longer column represents the growth in 
conditioned water. 

for two or more hours daily and are then washed 
off and placed in a fresh batch of water. Although 
the fish are never fed in the water they are condi- 
tioning, within a few hours after their transfer into it 
from the feeding aquarium the water becomes more 
or less cloudy with regurgitated food particles. These 
bits of food are large enough so that the growth-assay 


fishes can strain them out of the water. When such 
particles are removed by filtering, the growth-promot- 
ing power of the conditioned water is greatly les- 
sened, but it is not completely lost. In our experi- 
ments we found that suspended food particles ac- 
counted for 80 per cent or more of the increased 
growth in conditioned water over that given in clean 
control water. 

These experiments give certain suggestions con- 
cerning some other conditioning factors that may be 
acting. For example, we know that the skin glands of 
fish secrete slime (Figure 15). When we have made a 
chemical extract of this material we have frequently 
recovered a growth-promoting substance, apparently 
a protein, which was effective in stimulating growth 
when diluted 1 to 400,000, or even 1 to 800,000 times. 
At these dilutions it is not probable that this factor 
is affecting growth by furnishing food material. 

There are, of course, other possibilities, many of 
which we have checked. The increase in growth is 
not due, for example, to a change in the total salt 
content of the water, for this does not change in our 
experiments; nor to differences in acidity or oxygen, 
nor, so far as careful quantitative analyses have re- 
vealed, to changes in chemical elements present. We 
may be dealing with some sort of mass protection, 
such as was discussed in the last chapter, in which 



the conditioning fishes remove some harmful sub- 
stance, but of this we have no real evidence. 

Whatever the explanation, we are certain of the 
facts, and we know that we have demonstrated a de- 


NO. 56 59 


VS. ■■ P= 0.0106 



GROWTH 1,95 0.54 

NO. 61 122 




GROWTH 3.22 0.61 

P= 0.0006 

NO. 26 28 






GROWTH 1.92 1.55 

P= 0.26 

Fig. 15. An extract from the skin of goldfish fre- 
quently has growth-promoting power. The arrangement 
of the figure is on the same plan as was used in Fig. 14. 


vice such that if in nature one or a few fish in a 
group find plenty of food, apparently without will- 
ing to do so they regurgitate some food particles 
which are taken by others, a sort of automatic shar- 
ing. Again, in water that changes rapidly, such stag- 
nant-water fishes as goldfish, if present in numbers, 
are able to condition their environment, perhaps by 
the secretion of mucus, so that it becomes a more 
favorable place in which to live and grow. 

Perhaps I have lingered too long over this one 
case; I am so close to the facts and to the tactics used 
in collecting them that they may seem rnore interest- 
ing to me than they will ten years hence. We have 
run the same experiment with positive results with 
a few other species of fishes; and we have also found 
by experimentation that certain fish will regenerate 
tails that have been cut off if several are present in 
the same water more rapidly than if each is isolated. 
(112) The same is true for the young tadpoles of sala- 
manders, with which we have had experience. The 
explanation of the more rapid regeneration of such 
cut tails is probably relatively simple. The several 
animals together more readily bring the surrounding 
fresh water to approximately the salt content of the 
cut and regenerating tissues than can be done by a 
single animal placed in the same amount of water. 


This may not be the whole of the story but it is prob- 
ably a significant part of it. 

In both of these cases the additional growth of 
aquatic animals, which occurs as a result of the pres- 
ence of other animals of the same species, is produced 
in response to some sort of chemical which has been 
given off into the surrounding water. This may be 
nothing more than the unswallowing of surplus food 
by the conditioning fish. With animals whose tails 
have been freshly cut off the addition of salts to the 
water by the group may balance the osmotic tension 
at the cut surfaces and so favor re-growth. The excit- 
ing result of these studies lies in the suggestion that 
some less obvious growth-promoting substances may 
also be secreted into the surrounding water. 

Animal aggregations frequently produce physical 
as well as chemical changes, and while we are con- 
sidering the effect of numbers of animals present on 
the rate of growth of individuals it is interesting to 
examine one case in which growth-promotion appears 
to have been produced largely by changes in tempera- 
ture. Such an effect has been reported more than 
once; it is most simply illustrated in a warm-blooded 
animal, this time the white mouse. The experiment 
was first performed in Poland, but the causal factors 
were then only partially recognized. It has been re- 


peated in our laboratory where significant steps have 
been taken towards its further analysis. 

Vetulani, the original experimenter, (117) used 
closely inbred mice for his experimental animals. He 
measured the growth of males and females separately 
from the sixth and on through the twenty-second 
weeks of their lives. After rearrangement he followed 
them for ten weeks longer as a sort of control. Fresh 
food was supplied in abundance each day, and proper 
experimental conditions seem to have been main- 

Growth during the first sixteen weeks of the ex- 
periment is shown in the accompanying graphs (Fig- 
ure 16). All started off at approximately the same 
rate. After the fifth week of the experiment, however, 
it is clear that the isolated mice were growing most 
slowly, and they continued to do so as long as the 
experiment ran. The most rapid rate of growth was 
observed in those mice which were placed two to 
four per cage; those five to six per cage grew next 
best, and only slightly below these came those living 
nine to twelve per cage. 

Under the conditions of this experiment the iso- 
lated young mice were most handicapped, those most 
crowded were next, while those that were somewhat 
but not too crowded grew most rapidly. When the 
mice were rearranged for a continuing period of ten 

OK — I 1 1 1 1 1 1 1 1 t I I I I . I 

6 7 8 9 10 11121314 15 1G17 1819 20 2I22 
A;,'c in weeks 

Fig. i6. White mice grow faster in small groups than 
in large ones; they grow slowest . when isolated (solid 
line). (From Vetulani.) 


weeks the same relations held, showing that it was 
the state of aggregation rather than individual dif- 
ferences between mouse and mouse which was impor- 
tant in producing the differences in growth rates. 

Mr. Retzlaff, (105) the student who brought this 
work into our laboratory, tried first to repeat Vetu- 
lani's experiments in a room held at relatively high 
temperatures (29-30° C). Under these conditions he 
found that insofar as significant differences existed 
they showed that most rapid growth occurred with 
the isolated mice. When, however, he lowered the 
room temperature to about 16° C. he obtained the 
same general effect as reported by Vetulani. It would 
seem then that in this case the opportunity to keep 
warm in a chilling temperature is one of the main 
factors in promoting growth of the crowded, but not 
too crowded, animals. This conclusion is strength- 
ened by recent analyses of the temperature relations 
of mice, made by French physiologists, (30) which 
show that a mammal as small as a mouse has great 
difficulty in maintaining a constant temperature and 
rarely does so for extended periods of time. A change 
of external temperature from 30° to 18° C. will cause 
a lowering of 0.4° in the body temperature of a 
resting mouse. 

With such temperature lability it is easy to see that 
a few mice huddled together as is their habit could 


help each other maintain their internal temperatures, 
conserving energy for growth, while if isolated they 
must use much of their energy in keeping warm. 

Vetulani observed another factor at work. Some 
of his mice had lesions of the skin which they treated 
by licking. When these were in the head region they 
could only be treated by another individual. Some 
of his isolated mice had such lesions when at the end 
of the first experimental period they were re-grouped 
for further observation; these wounds were soon 
cured by their new nest mates. 

When one turns from studying the rate of growth 
of individuals to that of populations of these higher 
sexual animals, many of the same principles can be 
observed working as were outlined in the last chap- 
ter for the growth of asexual populations of proto- 
zoans in which overcrowding retards population 
growth, while optimal crowding, at least in many 
instances, favors it. 

With experimental populations of mice, for exam- 
ple, three long, laborious experiments made in Scot- 
land (36) and in Chicago (106) have indicated that, 
under the conditions tried, the least crowded mice 
reproduce most rapidly. The same holds true for the 
well-studied fruit-fly, Drosophila. (96) 

Neither with these flies nor with the mice is there 
any indication to date of a more rapid rate of repro- 


duction per female when more than the minimal 
pair is present. I have a strong suspicion, however, 
that one would get a more rapid rate of increase per 
number of animals involved if, instead of keeping 
the sexes equal in numbers, there were a ratio, let 
us say, of two females to one male. 

We do know that with Drosophila the greatest 
numbers are produced when the feeding surface is 
relatively great but not too great; (60) this result may 
be explained by the assumption that with too great 
space, or in other words, with too few flies present, 
wild yeasts or molds grow more rapidly than the 
Drosophila can keep under control. 

Another well-studied laboratory animal, the flour 
beetle, Triholium, under certain experimental con- 
ditions gives most rapid population growth at an 
intermediate population size rather than with too 
few or too many present. A study of data collected 
by Chapman showed that in a flour beetle's little 
world, a microcosm of thirty-two grams of flour, these 
beetles, during the early stages of population growth, 
reproduce most rapidly per female with two pairs 
present (Figure 17). Reproduction is more rapid 
when four pairs or even sixteen pairs are present, 
than if there is only one pair. (3) 

This work of Dr. Chapman's was done for another 
purpose. We took it for an indication of possibilities. 


and Dr. Thomas Park looked into the matter inde- 
pendently. (88) He found the situation very much 
as it had originally appeared to be. A Scotsman 
named Maclagen had a curiosity along the same line 












\ ^^^^^^^^^ 


\^ ^""X^ 



\ ^^frdays 

^ 5 





*"'••. ^ 





1 « I 1 1 

2 4 8 16 32 64 

/nitiaC population per ^2 Sms, of f/oar 

Fig. 17. Flour beetles reproduce more rapidly if more 
than one pair is present. 

and independently re-checked the whole matter with 
the same results. (77) Three separate workers in 
three different laboratories have now obtained essen- 
tially similar results with these same beetles, and the 
chances that all are mistaken are rather remote. 

One of them, Dr. Thomas Park, has proceeded to 
analyze the factors involved. (89) He finds that the 
results come from the interaction of two opposing 
tendencies. In the first place, adult beetles roam at 


random through their floury universe. They eat the 
flour, but they may also eat their own eggs as they 
encounter these on their travels. This habit of egg- 
eating tends to reduce the rate of population growth, 
the more so the denser the population. 

The second factor is the experimentally proven 
fact that up to a certain point copulation and suc- 
cessive re-copulation stimulate the female Tribolium 
beetles to lay more eggs, and eggs with a higher per- 
centage of fertility. Thus the more dense the beetle 
population the more rapid its rate of increase. The 
interaction of these two opposing tendencies results 
in an intermediate optimal population in which 
more offspring are produced per adult animal than 
in either more or less dense populations. 

It may be felt that I have been keeping too closely 
to the more or less artificial conditions found in the 
laboratory. It is true that in an attempt to bring the 
various aspects of the population problem under ex- 
perimental control we have avoided those field obser- 
vations which can only be recorded as more or less 
interesting anecdotes. We have now come to a point 
in our inquiry, however, at which it is necessary to 
move directly into the field. 

Given the evidence at hand, that optimal numbers 
present in a given situation have certain positive 
survival values and some definitely stimulating effects 


on the growth of individuals and the increase of 
populations, we strike the problem of the optimal 
size of a population in nature. This is an exceedingly 
difficult question on which to obtain data. Suppose, 
therefore, that we simplify it by asking what minimal 
numbers are necessary if a species is to maintain itself 
in nature? 

This inquiry is a direct attempt to find under nat- 
ural conditions the application of the statement by 
Professor Pearl that "this whole matter of influence 
of density of population in all senses, upon biological 
phenomena, deserves a great deal more attention 
than it has had. The indications all are that it is the 
most important and significant element in the bio- 
logical, as distinguished from the physical, environ- 
ment of organisms." 

Over and over again in the last half-dozen years 
I have asked field naturalists, students of birds, wild- 
life managers, anyone and everyone who might have 
had experience in that direction, how few members 
of a given species could maintain themselves in a 
given situation. Always until this last summer I have 
found that, stripped of extra verbiage behind which 
they might hide their ignorance, the real answer was 
that they did not know. 

And then I had two pieces of luck; I found a man 
and a scientific paper. My friend. Professor Phillips 


of South Africa, came to spend some weeks with us. 
He told us that the Knysna Forest, a protected wood- 
land in South Africa, has an area of 225 square miles, 
fifteen miles on a side, and that this forest is the 
home of a herd of eleven elephants, which can also 
range outside the forest limits. On the other hand, 
the Addo Forest, of twenty-five to thirty square miles, 
supports a herd of twenty-four elephants. (98) Dr. 
Phillips thinks that the smaller herd is not maintain- 
ing itself, and that the larger one, under apparently 
less favorable conditions as regards available area of 
range, is at approximately the lower limit for keep- 
ing up its own numbers. He estimates that an ele- 
phant herd of about twenty-five individuals could 
maintain itself in an unrestricted range providing 
civilized man were absent. 

He gave us a second example, of a herd of some 
three hundred springbok on a protected reserve of 
six thousand acres in the Transvaal, which was un- 
able to maintain its numbers and became reduced 
to eighty or ninety, on its way toward total ex- 

It is well known that in the life of equatorial 
Africa the tsetse fly plays an important part. It carries 
the trypanosomes which cause the deadly disease, 
"sleeping sickness," of man and his domestic animals, 
and which affect native game as well. The British 


colonial governments have been active in attempts 
to control the density of these fly populations. In 
general they are restricted to damp, low-lying forest. 
In districts where this is confined to the borders of 
water-courses, and hence where the fly belt has nat- 
urally a definite limit and is restricted in size, an 
ingenious fly trap has been used successfully. The 
trap takes advantage of the natural reactions of the 
tsetse fly. These are strongly positive to a slightly 
moving dark object a few feet above ground. With 
appropriate screening they can be caught as they fly 
toward such an object; they will fly up and fall back 
until they literally wear themselves out. It was at 
first thought that such a trap would be helpful chiefly 
in reducing the excess fly population; then, to the 
delight of the control officials, they found that when 
in these restricted fly belts the tsetse flies had been 
trapped down to a certain minimum population 
there was no need to catch the very last flies; below 
the minimum level those remaining disappeared 
spontaneously from the area. Nor did they return 
unless brought back in considerable numbers accom- 
panying movements of game, or as a result of the 
slow extension of range from other infested areas. 
The work of the control officials in such regions thus 
was very much easier than had been anticipated. 
Two pertinent cases concerning the minimum 


number below which a species cannot go with safety 
have come in part under my own observation. In 
1913, my first summer at the Marine Biological Lab- 
oratory at Woods Hole, Massachusetts, the veteran 
scientists of the laboratory, at least those who still 
were willing to exhibit naturalistic enthusiasms, were 
greatly pleased at the visit of a flock of laughing gulls 
to the Eel Pond near the laboratory. The main 
breeding ground of these gulls is on Muskeget Island 
off Nantucket. In 1850 the laughing gulls were abun- 
dant there; but they were exposed to the depreda- 
tions of egg takers and later, about 1876, to the 
attacks of men interested in obtaining their striking 
wings and other feathers to satisfy the millinery de- 
mand for feathers of native birds, which was then 
at its height. (49) Under this slaughter the colony 
was nearly wiped out; at its low point about 1880 
there were not more than twelve pairs of laughing 
gulls left on Muskeget Island, and only a few of these 
bred. A warden was employed in a somewhat extra- 
legal capacity by certain ornithologists who regretted 
seeing the species die out, and he was assisted by the 
captain of the local life-saving crew in protecting the 
gulls from raids. Later changes in laws regarding 
protection of birds and the use of plumage in mil- 
linery gave more secure protection for the growing 
colony. For the first ten years the birds increased 


slowly, but thereafter more rapidly, until there are 
now thousands breeding on the island, and their 
range has spread to the mainland. In Woods Hole, at 
the present time, these birds whose return in 1913 
excited so much comment are as common as the 
terns. In this case, a few breeding pairs, nesting in a 
relatively safe place, were able to regenerate the local 
population in less than fifty years; all that was needed 
was protection from the predations of man. 

The nesting colonies of gulls have attracted atten- 
tion from many; a report by Darling has recently 
appeared concerning certain relations between num- 
bers of herring gulls in a colony and breeding be- 
havior, and survival of young gulls on Priest Island 
off the northwest coast of Scotland. (39) There are 
indications that the members of larger colonies stim- 
ulate each other to begin mating activities earlier 
than when the colonies are smaller and, what is 
apparently more important, there tends to be a 
shorter spread in the time from the laying of the first 
egg until the last one is laid. This means that the 
breeding activities are more intense while they last. 

The period between hatching and the growth of 
the first adult plumage is a crucial time in the life 
of young gulls. While they are in the downy stage 
they are preyed upon by outside predators; also at 



this time the gull chicks that wander from their home 
nests may be pecked to death by other members of 
the colony. The toll of the chicks is comparatively 
less, the shorter the time from the hatching of the 



Fig. i8. The "spread" of time in which eggs are laid 
in a colony of herring gulls affects the percentage that 
survive. The smaller the colony the longer the spread, 
and the fewer survivors. [From Darling (39) by permis- 
sion of The Macmillan Co.] 

first fuzzy young gull until the last one changes to 
a young fledgling with adult feathers. These relations 
are graphically shown in Figure 18. 

Darling thinks that the greater success of the larger 
colonies does not lie in any vague factor of mutual 
protection, but in the nearer approach to simultane- 
ous breeding throughout the colony. This is a phase 
of social facilitation which will be discussed more 
fully in a later chapter. 

These observations need to be extended and con- 


firmed. They suggest one mechanism, that of mutual 
stimulation to mating, which may have operated to 
produce social nesting among birds, and which seems 
capable of giving added survival value to the larger 
colonies, once the habit of collecting into breeding 
flocks is established. We have here a suggestion that 
these social colonies of birds have evolved far enough 
so that there has come to be a threshold of numbers 
below which successful mating does not take place. 
The numbers that constitute this threshold probably 
vary under a variety of conditions. 

In one case, when only two pairs were present, 
nests were built but no eggs were laid, while in a 
more favorable season, with three pairs, eggs were 
laid and one chick out of eight that hatched lived 
through the downy stage. 

I saw the laughing gulls myself at Woods Hole 
last summer; and I also found a paper by Gross giv- 
ing the case of another almost extinct population 
which could not be revived. The heath hen, prob- 
ably a representative of an eastern race of the prairie 
chicken, was formerly very abundant in Massachu- 
setts, and may have been distributed from Maine to 
Delaware, or perhaps even further south. It was grad- 
ually isolated by the killing of birds in the intermedi- 
ate region and was driven back, until about 1850 it 
was found only on Martha's Vineyard and the near-by 


islands, and among the pine barrens of New Jersey. 
(56) By 1880, except for attempted and unsuccessful 
introductions elsewhere, it was probably restricted to 
Martha's Vineyard. In 1890-92 it was estimated that 
one hundred to two hundred birds remained on that 
island. Then several things happened at about the 
same time: prairie chickens were introduced and 
probably interbred with the vanishing heath hen, 
protection of the birds was stiffened, and collectors' 
prices went up! It is an interesting commentary that 
most of the museum specimens, of which 208 are 
known at present, were collected between 1891 and 
1900, when the probable extinction of the heath hen 
was noised abroad. This is one of the modern handi- 
caps of small numbers; let a species or race become 
known to be rare, and museum collectors feel it 
their special duty to get a good supply laid in, just 
in case it does become extinct. 

By 1907, when the Heath Hen Association was 
formed and employed a competent warden, the count 
had been reduced to seventy-seven. Massachusetts 
became aroused and purchased six hundred acres of 
heath hen range and leased a thousand acres more. 
The reservation was near a state forest which added 
another thousand acres of protected range. The birds 
responded to increased care and by 1916 it was esti- 
mated that there were two thousand in existence. 


Then came a fire, a gale, and a hard winter, with 
an unprecedented flight of goshawks, and in April, 
1917, there were fewer than fifty breeding pairs. The 
next year, when there was an estimated total popu- 
lation of 150, the heath hen range was invaded by 
several expert photographers who took motion pic- 
tures of mating behavior. In the face of this disturb- 
ance at a critical time, still a good year allowed the 
birds to increase and again to spread over Martha's 
Vineyard. In 1920, 314 were counted; but thereafter 
a decline in numbers set in which was never stopped. 
The figures for those five successive years are: 117, 
100, 28, 54, 25. At this point extra wardens were put 
on the job, who killed more cats, crows, rats, hawks, 
and owls, the enemies of the heath hen. The next 
year's count was 35; in 1927, there were 20; but in 
1928, in a census that lasted four days, only a single 
male was found. No other bird was seen thereafter, 
though a reward of a hundred dollars was offered 
for the discovery of another. This single male was 
banded and released and was last seen alive on Febru- 
ary 9, 1932. With his death the heath hen became 
extinct. (18) 

When this much is known of the decline in num- 
bers of a given species there should be some knowl- 
edge of the factors involved in its extinction. There 
is. In the earlier years, as I have indicated with re- 


gard to museum collecting, there was undoubtedly a 
considerable amount of poaching; but as population 
of heath hens declined, local sentiment turned in 
favor of protection and poaching decreased, both 
because of a more intelligent public reaction to the 
birds, and because of closer patrol by wardens. Dr. 
Gross, whose account I have been following, thinks 
that there was evidence of an inadaptability of the 
species, an excessive inbreeding, and, at the end, an 
excessive number of males. In such small populations 
the sex ratios frequently become highly abnormal. 
Disease and parasites took their toll. Predators, par- 
ticularly cats and rats, were active. The females hid 
their nests well and were faithful in remaining on 
them, so that they were killed off by the fires which 
at times whipped over the breeding grounds. 

Over sixty thousand dollars was spent in trying to 
save the heath hen, but without success. In contrast 
to the laughing gull, which nested in a relatively 
safe place and which came back from a population 
as low as the heath hen's until the very last, this 
unfortunate species was not able to adjust itself and 
continue existence, even with as intelligent human 
help as could be mustered in its favor. 

The general conclusion seems to be that different 
species have different minimum populations below 


which the species cannot go with safety, and that in 
some instances this is considerably above the theo- 
retical minimum of one pair. 

By way of the laboratory, the coastal regions of 
Massachusetts, and South African grassland and for- 
est, we are arriving at a general biological principle 
regarding the importance of numbers present on the 
growth, survival and, as we shall see, upon the evolu- 
tion of species of animals. 

Lacking definitive information on this last phase 
of the subject, we shall turn to mathematical explo- 
rations of its possibilities, as made primarily by Pro- 
fessor Sewall Wright. (127, 41) Although the ideas to 
be presented are essentially simple in principle, they 
are sufficiently novel and unfamiliar to challenge the 
closest attention. 

I shall not indulge here in the details of the mathe- 
matical analyses, for the very good reason that I do 
not understand them. If I were not convinced, how- 
ever, that Professor Wright does understand them I 
should not present this outline. It is only fair to say 
that, in my opinion, in dealing with these ideal popu- 
lations Professor Wright cannot bring into sharp 
focus at one time all the factors that may be acting 
in nature. This is what he Has been courageous 
enough to attempt; the more nearly he succeeds, the 
more likely is the calculation to be too complex for 


presentation in detail except to highly specialized 

The environment is in a state of constant flux and 
its progressive changes, whether slow or fast, make 
the well-adapted types of the past generations into 
misfits under present conditions. The result may be 
rectified either by the extinction of the species, if it 
is not sufficiently plastic, or through reorganization 
of the hereditary types. In such a reorganization the 
simple Lamarckian reactions apparently do not op- 
erate; that is to say, when confronted with new, 
critical conditions, species cannot go to work and 
produce needed changes to order. The reactions are 
much more complicated than that. 

To present the modern interpretation of this re- 
organization I need three technical terms which I 
shall define before using. Genes are bits of proto- 
plasm too small to be seen through the microscope, 
which are located in all cells and which are thought 
to be the bearers of heredity. They behave as indi- 
visible units, that is to say, a gene if present in an 
organism is either transmitted as a whole or not at 
all. Gene frequency is the term applied to the fre- 
quency with which a given gene is found in a popu- 
lation, relative to the total possible frequency (two 
in every individual). By mutation is meant a large or 
small hereditary change which appears suddenly, 


usually in the sense in which I shall use it, as a result 
of a change in one or more genes. With these three 
terms in mind we are ready to try to understand how 
the hereditary types may become reorganized. 

Such a reorganization implies a change in gene 
frequencies. By this I mean now that there will be a 
decrease in the abundance of the genes which were 
responsible for the past adaptations that are now 
obsolete, and an increase in the frequency of those 
genes which allow an adaptation to the new condi- 
tions. Gene frequencies remain constant in a large 
population unless changed by mutation, selection or 
immigration. This is because of the unitary charac- 
ter, without blending, and the symmetry of the 
Mendelian mechanism of heredity. 

These life-saving genes may have been present 
in the species for a million years as a result of 
long past mutations, without having been of any 
value to the species in all that time. Now under 
changed conditions they may save it from extinction. 
It is important to note that organisms do not usually 
meet changed conditions by waiting for a new muta- 
tion; frequently all members of a species would be 
dead long before the right change would occur. This 
means that since a species cannot produce adaptive 
changes when and where needed, in order to persist 


successfully it must possess at all times a store of 
concealed potential variability. 

I may interject parenthetically that at times this 
appears to call for the presence of a considerable 
number of individuals as a necessary condition to 
provide the needed variations. A part of this reserve 
of variability may be of no use under any circum- 
stances; some characters may be useful, some may 
never meet with the circumstances under which they 
would have survival value; while others, though of 
no use or even harmful when they appear, may later 
enable the species to live under newly changed con- 

Hereditary changes tend to be eliminated as soon 
as they run counter to decided environmental selec- 
tion. In large populations the results of mutations 
tend to stabilize about some average gene frequency, 
which represents the interaction between the rate of 
mutation and the degree of selection. Frequently 
mutation pressure pushes in one direction and selec- 
tion in another and the resulting gene frequency in 
the population represents a point or zone of equi- 
librium between these forces. In small populations 
which are not too small, selection between genes 
becomes relatively ineffective, and the gene fre- 
quencies drift at random over a wide range about a 
certain mean position. In very small breeding popu- 



lations, even though these may be small isolated 
colonies of a large widespread species, gene fre- 
quencies drift into fixation of one alternative or an- 
other more rapidly than they are changed by selec- 




Fig. 19. In small populations, genes drift into fixa- 
tion or loss largely irrespective of selection; the fre- 
quency of fixation or loss depends in the long run on the 
relative frequency of mutation and reverse mutation. 
(After Wright.) 

tion or by mutation. Mutation, however, prevents 
permanent fixation. The condition at any given 
moment is largely a matter of chance. 

Perhaps a diagram will help at this point. In Fig- 
ure 19 the horizontal axis shows the different gene 
frequencies in a population, and the vertical axis 
gives the chances of the population under considera- 
tion possessing any given gene frequency. At the left, 


the gene frequency is zero; that is, the gene in ques- 
tion is absent from the population for the time being. 
The height of the curve shows that there is a good 
chance of this happening. At the extreme right the 
gene has become fixed and all animals in the popu- 
lation have it; they are a pure culture so far as this 
gene is concerned. Again there is a high degree of 
probability that this may happen when numbers are 
few. But the intermediate condition, when the gene 
is present in some but not all of the animals, shows 
little chance of occurrence. 

In such small populations, as has been said before, 
the gene frequency is determined mainly by chance; 
any given hereditary unit tends to disappear com- 
pletely or become fixed and occur in all members of 
the small inbreeding colony. Such a condition may 
have been reached in the inbred population of the 
heath hen on Martha's Vineyard. 

With populations that are intermediate in size 
there is a greater variety of possibilities. Some genes 
are lost, others reach chance fixations, and others 
fluctuate widely in frequency from time to time. 
These conditions are shown in Figure 20. 

If a given species is isolated into breeding colonies 
in such a way that but little emigration occurs be- 
tween them, a condition known to exist in nature, in 
the course of time, as Professor Wright shows, the 


species will become divided into local races. This 
will happen although at the time of separation the 
populations were all homogeneous and the environ- 
ment of all remains essentially similar. 

If the environment does remain steady the larger 





0.5 100 

Fig. 20. In medium populations, gene frequencies 
drift at random about an intermediate point but not so 
much so that complete fixation or loss is likely to occur. 
(After Wright.) 

colonies will tend to keep the same hereditary consti- 
tution as that which the whole species formerly had. 
(Figure 21.) Small breeding colonies will, how- 
ever, become pure cultures for different characters, 
and it is impossible to predict the course of the 
hereditary drift in any of these populations. As illus- 
trated in Figure 20, the fixation will be a matter of 
chance, and local races will result without any neces- 
sary reference to adaptation. 

The snails in the different mountain valleys of 
Hawaii afford the classical illustration of this point. 



Each individual mountain valley has its separate 
species of snails. They are distinguished by size, by 
color markings, and by other characters which may 
be wholly non-adaptive. 

Colonies which are intermediate in size will pre- 





0.5 1.00 

Fig. 21. In large populations, gene frequency is held 
to a certain equilibrium value as a result of the oppos- 
ing pressures of mutation and selection. (After Wright.) 

serve a part of the variability that will be lost in the 
smaller colonies. Even so, there will be some inde- 
pendent drifting apart of the various gene frequen- 
cies, so that these, too, will give rise to new local 
races. Professor Wright's calculations show that with 
mutation rates of the order of i:io,ooo or 1:100,000, 
such intermediate populations, optimal for evolution, 
will consist of some thousands or tens of thousands of 

With small breeding populations, then, genes tend 
to become fixed or lost. Even rather severe selection 


is without effect. Individual genes drift from one 
state of fixation to another regardless of selection. In 
large populations, gene frequencies tend to come to 
equilibrium between mutation and selection, and if 
selection is severe, there tends to be a fixation of the 
gene or genes that carry adaptive modifications, and 
evolution comes to a standstill. 

With a population intermediate in size, when there 
are enough animals present to prevent fixation of 
the genes on the one hand, but on the other, not 
enough animals to prevent a random drifting about 
the mean values determined by selection and muta- 
tion, then evolution may occur relatively rapidly. 
The results obtained will depend upon the balance 
between mutation rate, selection rate, and the size 
of the effective breeding population. 

In one more case the effect of differences in sever- 
ity of selection was worked out by Professor Wright 
(Figure 22). With a moderate mutation rate, if the 
selection is relatively weak, mutation pressure may 
determine the result and the given character will 
then drift to fixation or, as shown in the diagram, to 
extinction. As selection pressures increase, selection 
tends to take charge of the end products, and, if 
slight, there is a wide variation about a mean; if 
more intense, the amount of variation becomes less 
and less. 



When a species is broken up into different breed- 
ing colonies, as it is with the snails in the Hawaiian 


Fig. 22. As intensity of selection increases it becomes 
more and more dominant in determining the end result, 
and the degree of variation is lessened; 4Ns gives selec- 
tion pressure. (From Wright.) 

valleys, (57) it can be similarly shown that the effects 
produced depend on the rate of emigration between 
colonies, as well as selection pressure, mutation pres- 
sure, and population size, other factors being con- 
stant. Cross-breeding introduces genes into a popula- 


tion in a way that is essentially identical with muta- 
tion in its mathematical consequences; however, 
similar results may be obtained in a much shorter 
time by cross-breeding. And in fact all the different 
results which have just been illustrated can be dupli- 
cated by varying the numbers of the emigrants. 

This is not the place to explore all the implica- 
tions and possibilities of these interesting analyses. 
The highly significant conclusion has been reached 
that if a species occurs not as a single breeding unit 
but broken into effective breeding colonies which 
are almost isolated from each other, the members of 
different colonies, given sufficient vigor, may evolve 
into dissimilar local races. If one of these becomes 
well adapted to its environment it may increase in 
numbers and send out numerous emigrants. If these 
emigrants find and interbreed with members of other 
less advanced colonies they will grade these up until 
they resemble the most adapted colony. This part of 
the process resembles a stock breeder's grading up 
of a mediocre herd of cattle by repeated infusions of 
new and improved ''blood" into his herd. The sig- 
nificant thing here is that the random differentiation 
of local populations furnishes material for the action 
of selection on types as wholes, rather than on the 
mere average adaptive effects of individual genes. 

The end results will vary even when the original 


population was homogeneous, and when mutation 
rates are similar throughout, even though selection is 
in the same direction in all parts of the different 
colonies. The primary factor under these conditions 
will be that of effective breeding population size, and 
there will be greater chance for varied evolution 
among the populations that are intermediate in size, 
as contrasted with those which are small or large, and 
still greater chance for evolution when a large species 
is broken into small breeding colonies which are not 
completely isolated from each other. 

This argument, even as I have simplified it, is not 
too easily followed the first time one goes over it. 
Perhaps my use of an old teaching trick, that of repe- 
tition of the same ideas with different words and 
different illustrations, may be forgiven. In doing so 
I am still leaning heavily on Professor Wright. The 
series of diagrams shown in Plate IV are built on 
one fundamental background. In perspective we see 
two elevations, one higher than the other, and two 
depressions which are the low points in a valley 
between the two peaks. Every position is intended 
to represent a different combination of gene fre- 
quencies. The peaks represent gene combinations 
which are highly adaptive; the depressions represent 
those that lack adaptive value. The degree of adap- 
tiveness is shown by the height occupied by the given 




.X- O 


PLATE IV. A population originally possessed a set 
of gene combinations of some slight adaptive value 
(dotted line). With increased mutation rate it can ex- 
pand to less adapted levels (A); with increased selec- 
tion it contracts (B); if the environment changes the 
gene frequency must shift (C); with small numbers 
and close inbreeding the course of evolution is erratic 
and extinction usually follows (D); with larger num- 
bers, evolution takes place more readily (E); most read- 
ily, when a large population is broken into local 
colonies with inter-emigiation (F). (Modified from 


population. The variability of the population is 
shown by the size of the area that is occupied. Every 
individual in a species may have a different gene 
combination from every other, and yet the species 
may occupy a small region relative to all the possi- 

We may call the lower peak Mount Minor Adap- 
tation and the higher one Mount Major Adaptation. 
In Figure A we find a population which is fairly 
well-adapted, but not so much so as if it occupied the 
higher peak. Its original position and its variability 
are shown by the dotted circle. As a result of increased 
rate of mutation or of reduced selection, or both, the 
variability of the population has increased and it 
now spreads down to lower positions on this Mount 
Minor Adaptation. It contains more aberrant indi- 
viduals and even freaks than when subject to less 
frequent mutation or to more severe selection, and a 
freak may appear that is more adaptive; but this 
important end has been achieved at the expense of 
the variability which might have made a major ad- 
vance possible. 

Figure C introduces a different situation. As a 
result of environmental change Mount Minor Adap- 
tation has disappeared and the adapted population 
has been able to move to a new location at about the 
same level formerly occupied; now it is on the slope 


of Mount Major Adaptation, and if selection con- 
tinues may be expected to move up that adaptive 
peak. A continually changing environment is un- 
doubtedly an important factor in evolution. 

The effects of population size are illustrated in the 
next three diagrams. The general background is the 
same as in Figures A and B. In Figure D is shown 
the effect of a decided reduction in population size, 
and consequently in variability, in the species that 
formerly occupied Mount Minor Adaptation. It is 
in fact so small that selection has become ineffective 
and the different hereditary qualities shift to chance 
fixations. As non-adaptive characters become fixed at 
random the species moves down from its peak over 
an erratic, unpredictable path. With reduction of 
population size below a certain minimum, control by 
selection between genes disappears to such an extent 
that the end can only be extinction. 

With the species population intermediate in size, 
with the same mutation and selection rates as before, 
gene frequencies move about at random but without 
reaching the degree of fixation found in the preced- 
ing case. Since it will be easier to escape from low 
adaptive peaks, the population will tend finally to 
occupy the more adapted levels. The rate of progress 
is, however, extremely slow. 

Finally, in Figure F, we see the case of a large 


species which has become broken up into many small 
local races, perhaps as a result of restricted environ- 
mental niches. Each of these local races breeds largely 
within its own colony, but there is an occasional emi- 
gration from one to another. Each tends, if it is small 
in number, to give rise to different variations which 
shift about in a non-adaptive manner. The total 
number of relatively stable variations will be much 
greater since the total number of individuals is so 
much larger than in E. Under these conditions the 
chances are good that some of the local colonies will 
escape from the influence of Mount Minor Adapta- 
tion and manage to cross the valley to Mount Major 
Adaptation. Here the race will expand in numbers 
and will send out more and more emigrants which 
will interbreed with the stocks in the less adapted 
colonies and tend to grade them all up toward a 
higher adaptive level. 

The conclusion is as Professor Wright says: "A 
subdivision of a large species into numerous small, 
partially isolated races gives the most effective setting 
for the operation of the trial and error mechanism 
in the field of evolution that results from gene com- 

In the rate of evolution, therefore, population size 
is as important as we have seen it to be in the growth 


of individuals or in the gTO\\ih of popnlation num- 
bers: and the optimal population size does not coin- 
tide \sith either the largest or smallest possible but 
lies at some iiuermediate point. 


Group Behavior 

IN THE second chapter I told of how I stumbled on 
the fact that in the breeding season the normal be- 
havior of isopods is affected by numbers present. 
Such effects have long been known for many types 
of behavior, and it would not be profitable here to 
catalogue and analyze all the cases that are on record. 
Rather, as before, I shall select certain well-authenti- 
cated examples of breeding reactions and of other 
types of behavior. Those which are chosen are espe- 
cially noteworthy because of the behavior pattern 
which is involved, or because freshly observed, or 

And here is a shift in emphasis. I have been stress- 
ing the existence of a widespread, fundamental auto- 
matic co-operation which has survival value, and have 
given evidence that it is a common trait in the animal 
kingdom. In this chapter I shall discuss group be- 
havior which may or may not have immediate sur- 
vival value. In each instance, and throughout the 
discussion as a whole, I shall be engaged in trying 



to find to what extent behavior is influenced by the 
presence of others, and shall not consistently attempt 
to assay possible values which may or may not be 

With many more or less social animals the group 
up to a certain size facilitates various types of be- 
havior. This is frequently called social facilitation. 

Shore Line 

Fig. 23. Manakin males establish rows of mating courts 
in the Panamanian rain-forest. (From Chapman.) 

One phase of social facilitation is illustrated by some 
observations of the mature student of birds, Frank E. 
Chapman, (28) near the tropical laboratory on Barro 
Colorado Island in the rain-forest of Panama. Mr. 
Chapman found that males of Gould's manakin 
establish lines of courting places (Figure 23). The 
manakin is a small warbler-like bird, delicately 
colored and relatively inconspicuous. Each of the 
courting places is occupied by a single male; the line 
thus formed extends for many yards through the 
undergrowth of the rain-forest. From time to time 
each day during the long nesting season, the males 
resort to their individual cleared spots on the forest 


floor and make their presence known by a series of 
snaps, whirrs and calls which may be heard as far as 
three hundred yards. The females, who are more 
quiet and retiring, apparently are attracted by the 
line of males; they come individually from the sur- 
rounding thickets and each mates with one of the 
males. The evidence suggests that they are attracted 
from a greater distance by the spaced aggregation of 
males than they would be by isolated courting places. 
The more or less organized line of males in breeding 
condition apparently facilitates the mating of these 
jungle birds. 

This is a highly specialized example of the wide- 
spread phenomenon of territoriality which can be 
recognized even among breeding fishes, (103) and 
which has been much studied of recent years in birds. 
(65) Typically the male birds arrive first in the 
spring and take up fairly well-defined territories in 
the same general area, which they defend from in- 
truding males. Then the females come in and flit 
from territory to territory before settling down to 
raise a brood with one particular male. There is 
always the strong suggestion that the presence of a 
number of singing males, even if spaced about in 
different territories, attracts and hastens the accept- 
ance of some one of them by an unmated female. 

Group stimulation of the amount of food taken 



has been reported for various animals, including 
rats, (59) chickens (23) and fishes. (118) I shall illus- 
trate by some of the experiments conducted in our 
laboratory by Dr. J. C. Welty. These have been 





^ 75 






le 25 





Fig. 24. Many kinds of fishes eat more if several are 
present than if they are isolated. (From Welty.) 

amply verified by other research workers. In connec- 
tion with experiments on the effect of numbers on 
the rate of learning in fishes, which will be discussed 
later. Dr. Welty undertook to find whether grouped 
fish ate more or less than if they were isolated. The 
results of a typical experiment are illustrated in 
Figure 24. 

Goldfish were photographed to scale, and those of 


similar size were selected for experimentation. Two 
groups of four each were placed in separate crystal- 
lizing dishes and eight others were isolated each into 
a wholly similar dish. The different dishes were sep- 
arated by black paper so that vision from one to the 
other was impossible. A known number of the small 
crustacean, Daphnia, were introduced daily into each 
dish. These living Daphnia had been screened so as 
to select the large animals only. As shown by the fig- 
ure, fish in all groups of four ate decidedly more on 
the first three days of the experiment. At this time 
the two lots were shifted. Those that had been 
grouped were now isolated, and vice versa. There 
was an immediate shift in the numbers of Daphnia 
taken, with the newly isolated animals now eating 
less than the accompanying groups. This indicates 
that we are dealing with an effect of numbers present 
rather than with chance differences in individual 
appetites. This difference kept up steadily until the 
last three days of observation, when an interesting 
complication arose. By this time the grouped fish 
were receiving a total of over six hundred Daphnia 
daily, including those which were eaten and the 
extras added to insure an economy of plenty. Each 
isolated fish was receiving only one-fourth as many. 
Now six hundred and more large Daphnia, each 
about an eighth of an inch long, make quite a swarm 


in a none-too-large crystallizing dish. The consump- 
tion of food per animal by the grouped fish fell off, 
and as was shown by appropriate tests, this was due to 
the action of a so-called confusion effect. When fewer 
Daphnia were present, a fish might be observed to 
swim after an isolated crustacean and eat it, whereas 
a dozen Daphnia or so in the immediate field of 
vision seemed to offer conflicting stimuli that blocked 
the feeding response. Working on this suggestion, 
one group of four was given the usual quota of some 
six hundred Daphnia all at once; another group was 
given only one hundred at a time, and when these 
were approximately all eaten then another hundred 
would fjc introduced, and so on until the end of the 
regular feeding period. This prevented the Daphnia 
from being too dense at the beginning of the hour's 
feeding time. The isolated fish were fed as usual. 
Under these conditions the grouped goldfish which 
were fed one hundred Daphnia at a time ate defi- 
nitely more than those given the whole confusing 
mass at once. 

Here we come upon two, not one, mass effects. In 
the first place we see that the fish in groups of four 
were stimulated to eat more food than if isolated, 
and this depended on their state of aggregation. But, 
incidental to this demonstration, we hnd that in the 
presence of too many animated food particles a con- 


fusion effect arises which decreases the feeding effi- 
ciency of the fish. 

It has been suspected for years that such a confu- 
sion effect exists and has survival value for small 
animals flocking together in the presence of a preda- 
tor, such as small birds in the region of a hawk. 
These observations of Welty's make the best demon- 
stration that I know of the existence of such an 
effect, in this case the Daphnia in the presence of the 
fish. I am less interested in this confusion effect at 
present than in the demonstration of social facilita- 
tion in feeding, a phenomenon which has been 
shown to exist for a number of fishes, including zebra 
fish, paradise fish, goldfish and guppies of the more 
usual aquarium varieties, and the lake shiner, No- 
tropis atherinoideSy as well. 

None of these fishes is very social, that is, none 
of them group into close schools. For evidence of 
similar social stimulation among social animals it is 
interesting to examine the effect of numbers present 
on the digging behavior of the highly social ants. 
The account of this work was published in 1937 by 
Professor Chen of Peiping, China. (29) 

These ants, a species of Campanotus, dig their 
nests in the ground. It was found that all the worker 
ants of this species are capable of digging a nest 
when in isolation, but that the rate of work varies 


with different individuals. If marked ants, whose 
reaction time has been tested in isolation, are placed 
together in pairs or in groups, they will start work 
sooner and will work with greater uniformity than 
if alone. 

With oriental patience. Professor Chen and his 
assistants collected and counted the number of the 
tiny pellets of earth which were dug by different 
individual ants when isolated, and when members 
of groups of two or three ants. They found that the 
number of pellets removed is greater when the ants 
work in association with others than when each 
works alone. This accelerating effect is greater for 
slow than for rapid workers; when ants with inter- 
mediate working tendencies were tested (Figure 25) 
they were found to be speeded up when in com- 
pany with a rapid co-worker and relatively retarded 
when placed with a slowly working ant. Interestingly 
enough, there was no difference between the stimu- 
lating effect of one additional ant and of many ants 
on the rate of work of a given individual. The social 
facilitation seemed maximum for these digging tests 
when only a second individual was present. 

Ants which regularly work rapidly were found to 
be physiologically different from those that work 
more slowly. The faster workers were more suscepti- 
ble to starvation, to drying, and to exposure to ether 

5 ra e 20 25 30 35 40 45 50 55 60 5 10 15 20 25 30 35 40 45 50 55 60 

Fig. 25. An ant which works at an intermediate rate 
(Ml) may be speeded up if placed with an ant which 
works more rapidly (Rl) and slowed down if put with a 
slower worker (SI). (From Chen.) 


or to chloroform. Tests that have been made by 
others indicate that animals that are more active 
physiologically usually succumb sooner under such 
adverse conditions, just as these rapidly-working ants 
were found to do. These are exceedingly interesting 
results because here we see that ants with apparently 
innate differences in speed of fundamental processes 
are affected in their speed of digging by the presence 
or the absence of a nest mate. The ant of intermediate 
speed, presumably with an intermediate underlying 
reaction system, is most interesting of all, because it 
can be either speeded up or retarded according as it 
is placed with an active or a more passive individual. 

In this connection it has been known for over a 
decade scientifically what was common sense before 
that time, namely, that human animals, whether 
adults or children, can accomplish more mental and 
physical work, at least of certain kinds, and will work 
with greater uniformity when in association with 
others doing similar tasks, than if obliged to work in 
isolation. (15, 84) 

Such considerations lead directly to problems con- 
cerning the effect of numbers present on the rate of 
learning in man. Here we find a set of questions that 
have great and immediate human significance. The 
world over, the training of the young animals of 
their own species is one of the major preoccupations 


of mankind. This is particularly true in the United 
States, where we are engaged in mass education on 
an unprecedented scale. This teaching of the young 
to the extent to which we are attempting it is an 
expensive business in time, in effort and in money. 
We need to know, therefore, the number of these 
interesting young animals that can be trained to- 
gether with best results. In other words, what is the 
optimal class size for the various levels of training 
from pre-school days through the preparation for 
the doctor's degree and further? 

In part, the proper answer to this question calls 
for a statement of educational objectives. The devel- 
opment of strong individuality, for example, is not 
necessarily accomplished by the same teaching meth- 
ods and class size which favor the growth of conform- 
ity to group patterns; and the rapid development of 
mastery of so-called skills may call for difiEerent num- 
ber relations than those needed for the mastery of 
logical thought. 

Even without positive information we can guess 
that the tutorial method with individuals or very 
small groups will best serve some ends while others 
will be achieved most readily in larger groups. The 
question, or a simplified part of it, thus becomes: 
What class size favors optimal rate of learning of the 
usual class material presented at different ages? 


As might be anticipated, the difficulties of human 
experimentation being what they are, it is hard to 
collect accurate information on this point. Much 
depends on the comparative accuracy of the sam- 
pling, and also on more subjective factors, such as 
the attitude of the teacher and of the students toward 
large and small classes. There is also a factor which I 
have not seen mentioned in the literature on the 
subject, the effect on the student of realizing or sus- 
pecting that he is an object of experimental interest, 
an educational guinea pig. This stimulus is more 
likely to be potent, in my opinion, when the student 
is a member of a class which is unusual in size. 

In the more careful studies, results of which have 
been published, the class numbers have ranged from 
"small" through ''medium" to "large." The "small" 
experimental classes apparently have about twenty 
to twenty-five members; this represents a more usual 
experience to the student, and he is more likely to 
be conscious of class size when he is a member of a 
large class of seventy-five or more than when he is in 
a small class or a medium-sized one of thirty-five to 
forty. The sizes that are counted "large" or "small" 
vary greatly, sometimes in the same experimental 
treatment, so that frequently the comparisons are 
between larger and smaller classes, both medium in 
size, rather than between real extremes in numbers. 


Frequently, too, the teaching practice varies in 
the two classes. Thus in one experiment the smaller 
classes in high-school geometry contained about 
twenty-five, while the large ones had about one 
hundred members. In the large classes a student 
helper was present for every ten class members. 
These helpers were superior students in geometry of 
the preceding year. As nearly as I can discover, there 
were no student helpers in the small classes. Under 
the conditions it is perhaps not unexpected that a 
better showing was made by those in the large classes. 
With them, there were present not only more in- 
structors per student but these were people of nearly 
their own age, who could be approached without 
hesitation not only in class but out of class and even 
out of school hours. Every mature teacher knows 
that even with the best intention and the most demo- 
cratic attitude, age differences widen the gap between 
the teacher and the taught, whatever other compen- 
sations there may be. 

The most comprehensive experiments I have seen 
reported in this field are those of the sub-committee 
on class size of the committee on educational re- 
search at the University of Minnesota. (66) These 
were carried on at the college level and involved 109 
classes under twenty-one instructors in eleven de- 
partments of four colleges in the University of Min- 


nesota. Forty-two hundred and five students were 
observed in large classes, and 1,854 in small ones; 
of these 1,288 were paired as to intelligence, sex 
and scholarship before the experiment began. One of 
each pair was assigned to a large and one to a small 
class in the same subject taught by the same instruc- 
tor. In this way the obvious variables were controlled 
as well as is humanly possible, unless we could have 
a large number of identical twins with which to 

In 78 per cent of the experiments a more or less 
decided advantage accrued to the paired students in 
the large classes, and at every scholarship level tested, 
the paired students in the large sections did better 
work than their pairs in the smaller ones; the excel- 
lent students appeared to profit somewhat more from 
being in large classes than their less outstanding 

Of the available data, a re-examination of the sum- 
maries indicates that there is on the average a dif- 
ference in the means in the final grade of 4.1 points, 
favoring the students in the larger classes. There 
is a statistical probability of matching this by 
random sampling of four chances in ten million 
(P = 0.0000004), and this despite the fact that the 
majority of the class comparisons did not give signifi- 
cant differences when considered alone. 


The numbers in the smaller classes usually ranged 
from twenty-one to thirty, but in some classes 
dropped as low as twelve; in the larger classes there 
were usually thirty -five to seventy-nine students; in 
the largest, one hundred and sixty-nine. Under the 
conditions which prevailed in these classes in psy- 
chology, educational psychology and physics, the stu- 
dents in the larger class sections made slightly but 
significantly higher final grades than those in smaller 
sections of the same subject taught by the same 

So much for objective experiments. It happens 
that subjective estimates, made both by teachers and 
by students at Minnesota, favor the smaller rather 
than the larger classes. It was even true that the 
students were better satisfied with the marks re- 
ceived in smaller classes than they were with the 
slightly higher grades given them in the larger sec- 

The general attitude seemed somewhat like that 
toward a friend of mine who teaches general mathe- 
matics at Purdue University. He is an experienced 
and excellent teacher. His program for one semester 
required that he should meet a normal-sized class 
of thirty to thirty-five at eight o'clock, and that at 
nine o'clock he should meet a class of double the size 
in a larger room, to repeat the same subject matter. 


At the close of the semester the two sections were 
asked to rank the instructor on many different points. 
Uniformly the students in the larger section rated 
him lower than those in the smaller section, in such 
matters as teaching skill, pleasantness of voice, neat- 
ness of appearance and personal attractiveness! 

I have had a fairly extensive teaching experience, 
which has included work in grade- and high-school 
teaching, as well as over twenty-five years of teaching 
at the college and university level, during which time 
I have taught classes of almost all sizes, from those 
of over six hundred at the University of California 
to the graduate classes of three or four that come my 
way; and I must confess to a personal prejudice 
against these very large classes. Even when using the 
same lecture notes, I do not give the same lecture to 
five hundred students that I give to forty or fifty. 
On the other hand, even with graduate classes and 
advanced seminars I am prejudiced in favor of hav- 
ing enough students, which means at least eight to 
ten, to give a certain esprit de corps to the group. 
Such personal opinions have their value, particularly 
when they click with experimental results such as 
those outlined by Hudelson from the experiments 
at Minnesota. It is unfortunate that those experi- 
ments did not test either the upper or the lower 
limits of class size which are conducive to good class- 


room performance on the part of the students; and I 
know of none that does test these points adequately. 

Some of the difficulties which are inherent in ex- 
perimentation on the effects of class size on the rate 
of learning in man can be obviated by the use of 
non-human animals. This procedure does not solve 
all the requirements for elegant objective experimen- 
tation, and has the additional real difficulty of elim- 
inating all possibility of adding subjective impres- 
sions to objective findings, a point which makes one 
of the strongest arguments for experimentation on 
man when feasible. 

In some respects the most completely controlled 
experiments on the effect of numbers present on 
the rate of learning are those that Miss Gates and I 
performed some years ago, using common cock- 
roaches as experimental animals. (52) Earlier work 
by two independent investigators had shown that 
cockroaches can be trained to run a simple maze, 
and can show improvement from day to day. In our 
experiments we found that the cockroaches could be 
trained to run the maze we used by fifteen to twenty- 
five successive trials on a given day, and showed defi- 
nite improvement both in time taken to run the 
maze and in number of errors. However, unlike the 
experience of our predecessors, these University of 


Chicago cockroaches could not carry over the effects 
of training from one day to the next. 

The reason for this difference between our cock- 
roaches and those around St. Louis and in Germany 
is not known. It may be that at the University of 
Chicago, despite our reputation for scholarship, the 
local cockroaches have a low IQ, or it may be that 
since we used animals from the bacteriological lab- 
oratory, because of their unusual size and physical 
vigor, we were unconsciously selecting the dumber 
sort. Or perhaps, contrary to our plan, we set them a 
problem which is intrinsically more difficult for the 
cockroach mentality. In any event, it is important to 
remember that our cockroaches forgot overnight any- 
thing they may have learned the day before. As it 
turns out, this was fortunate for the experiments we 
were carrying on, because we could match up indi- 
vidual cockroaches with the same speed of learning 
in pairs or groups of three for later tests without fear 
of a carryover from their previous experience. 

The maze used is shown in Figure 26. It consisted 
of a metal platform from which three runways ex- 
tended, each about two inches wide and a foot or so 
long. The two side runways ended blindly, but the 
center one led to a black bottle, which allowed the 
cockroaches to escape from the light. This apparently 


was a reward for cockroaches which, when possible, 
give a negative reaction to light. 

The three-pronged set of runways was mounted 
about half an inch above a pan of water, which the 
majority of the cockroaches tended to avoid, and so 
kept on the runways. The tests were all made in a 

Fig. 26. A simple maze used in training cockroaches. 

dark room and light was furnished by a single elec- 
tric bulb mounted just above the point where the 
central runway left the main platform. In other 
words, the cockroaches, which are negative to light, 
had to learn to run through the area of strongest 
illumination in order to reach the dark bottle which 
served as a reward. After two minutes' rest in the 
dark bottle the cockroaches were literally poured out 
onto the platform of the maze without being touched 
by the experimenter, and observation of them began 

The problem as set was about at the limit of cock- 



roach ability. Approximately one-third of the insects 
tested never learned to stay on the maze; whenever 
they were placed on it they proceeded immediately to 
run off into the underlying water. Of the two-thirds 





isoIa."tecL * 
J roup of 3 




— r- 


Fig. 27. Isolated cockroaches make fewer errors on 
the maze than the same animals paired, and still fewer 
than if three are being trained together. 



that did learn to remain on the maze, a half, or an- 
other third of all those tested, did not show improve- 
ment in speed of reaching the bottle, after repeated 


isola-tecL •— * 



^roup of 3 •—• 



t (>■ 

f 5- 





^ . 







Fig. 28. They also take less time. 

trials. Thus only one-third of the cockroaches we 
tested showed improvement with experience, and, 
as I said before, they forgot overnight all that they 
learned during the day. 

As shown in the summarizing graphs (Figures 27 
and 28), isolated cockroaches made fewer errors per 
trial throughout the whole training period. They 


also took less time to run the maze than when the 
same animals were members of pairs or of groups of 
three. Turning the comparison around, paired cock- 
roaches took longer time per trial and made more 
errors than when isolated, and groups of three took 
still longer and made more errors than those in pairs. 

A study of the rate of improvement shows that 
during the early part of the training, as is indicated 
by the slant of the graphs, so far as time spent is 
concerned, paired cockroaches improved more rap- 
idly than they did if isolated or in groups of three, 
and those placed three together on the maze im- 
proved somewhat more rapidly than they did when 
isolated. Thus, while the presence of one or two extra 
cockroaches slowed down the speed of reaction on the 
maze and increased the number of errors made at all 
times, yet the rate of improvement in speed of re- 
action was higher when more than one was present. 
There was, however, no significant difference in rate 
of improvement as measured by number of errors. 

Excluding this one aspect of rate of improvement 
in time spent on the maze, in all other phases of 
the experiment isolated cockroaches turned in a bet- 
ter learning performance than they showed when 
more were present. Evidently under the conditions 
of our experiments the tutorial system usually works 
best with cockroaches. 


Essentially the same sort of experiment was tried 
with isolated and paired Australian parrakeets, which 
are commonly called love birds. (11) Rather naively, 
perhaps, I thought that since these birds so readily 
pair off, perhaps two might learn to run a simple 
maze more rapidly than a single individual would. 
This turned out to be entirely a mistaken idea. I 
shall spare you the details concerning this maze; it 
was adequate in size, so that two birds could pass 
through practically abreast. Almost all the ninety- 
odd birds that were tested learned easily to run the 
maze and normally reduced their time per trial from 
about two minutes to a few seconds, after six or 
seven days of training. Errors also were reduced, and 
several of the birds were trained so that they ran 
the maze day after day with no errors at all. 

The selected summarizing graphs (Figures 29 and 
30) will outline the results obtained. It made no dif- 
ference whether the birds were caged in pairs or 
separately; if placed alone in the maze the perform- 
ance was similar. If, however, two birds were put 
together in the maze, the speed was reduced and 
errors increased as compared with the scores made 
by isolated parrakeets. It made no difference whether 
two males, two females or a male and a female were 
trained in the maze together; there was always in- 
terference. The tendency was for the more rapid 


bird to slow down rather than for the slower bird 
to speed up. The paired birds tended to take the 
same time and to make the same errors. Given suf- 
ficient training, they might make perfect scores so 


Fig. 29. Parrakeets learn equally well if trained when 
isolated, whether they are caged singly or in pairs. A, 
time per trial; B, errors per trial. 



far as errors were concerned, but even after long 
training the performance of pairs was always more 

Fig. 30. Parrakeets learn more rapidly if trained 
alone than if two are placed together in the maze. A, 
time per trial; B, errors per trial, (The upper curve is 
unsmoothed; the lower three have been smoothed mathe- 

erratic than that of isolated birds. When birds that 
had been trained to a consistent level of excellence 
were exchanged so that those formerly isolated were 
paired and those formerly paired were isolated, their 
behavior in the maze took on the characteristics 


usually shown by paired and by isolated birds, prov- 
ing that the type of reaction given was a result of 
the numbers present rather than of the working of 
other factors. With these love birds then, contrary 
to the original assumption, all indications were that 
being paired in the maze slowed down the rate of 
learning and increased the erratic character of their 

Our experience with the general problem did not 
end here. I teach at the University of Chicago a 
favorite course called Animal Behavior. In this class 
the beginning research students attempt some small 
problem and frequently make good progress toward 
its superficial solution. One of these student projects 
has been the training of the common mud-minnow 
to react to traffic lights. The fish were trained to 
jump out of water and obtain a bit of earthworm 
when red was flashed. Under the green light they 
were conditioned to retire to one of the bottom cor- 
ners. If they did jump under green light they were 
fed filter paper soaked in turpentine. Within two 
months a lot of fishes, isolated one in each small 
aquarium, could be trained so that they would have 
been given an A for the project if they had been 
properly enrolled students. 

When, however, several fishes were placed together 
in the same aquarium and an attempt was made to 


train all at the same time the rate of learning was 
retarded. Paired fish reacted as well as if they had 
been isolated, but the reactions of groups of four 
were slowed down, and those of ten even more so. 
Two fish would rarely jump at once, and when some 
one individual was getting set to jump for the food 
under the red light, another would frequently come 
along and give him a jab in the belly which would 
stop all tendency to jump for the time. 

One more instance remains to be reported. Dr. 
Welty, who has been mentioned before, undertook 
to train goldfish to move forward from the rear 
screened-off portion of an aquarium through a door 
into a small forward chamber where each was fed 
just after it came through the opening. (118) An 
aquarium-maze, similar to those used, is shown in Fig- 
ure 31. The signal to the fish that it was time to 
react came from increasing the intensity of light in 
the aquarium and opening the door between the two 
compartments. Under Dr. Welty's careful coaching 
the fish improved rapidly in their speed of reaction 
and usually had reached a good level of performance 
by the sixth day of training. 

In his experiments almost a thousand fishes were 
trained at one time or another. The results of a 
sample experiment are recorded in Figure 32. In this 
test there were eight goldfish, each isolated in indi- 



vidual aquaria, four sets of paired goldfish, two lots 
of four placed together, and one group of eight in 
one aquarium. As shown by the graph, there was a 

Fig. 31. Feeding a fish which has just come through 
the opening from the larger side of the aquarium. (From 

marked group effect on the rate of learning. The 
speed of first performance of the untrained fishes was 
most rapid with eight present and slowest with iso- 
lated goldfish. In the early days of rapid learning the 
same order held. This experiment was repeated sev- 
eral times with identical results. Under these condi- 
tions there seems to be little doubt but that the 







TRIAL 5 10 14 

Fig. 32. Goldfish learn to swim a simple aquarium- 
maze the more readily the more fish there are present. 
(From Welty.) 



groups of goldfish learned to move forward and se- 
cure food more rapidly than the same number of 
isolated fish. 

The conditions of the experiments allow certain 


Fig. 33. Isolated goldfish learn the problem set for 
them less rapidly, and unlearn it more readily. (From 

types of analyses to be made. One of these is to test 
the tenacity with which the newly acquired habit 
will be retained. A set of fish was trained as usual 
(Figure 33). After ten days, when the grouped fish 
had been letter-perfect for four days, although the 
isolated goldfish were still taking some three min- 
utes per trial, the experiment was changed; when- 


ever the fish came forward through the gate they 
were offered pieces of worm soaked in acetic acid. 
The isolated fish, perhaps because they had not 
learned to perform so well, perhaps because they 
were isolated or for some other reason, ceased to 
react rapidly, and on the twenty-ninth day they were 
averaging fifteen minutes per trial. The grouped 
fish were much more steady in behavior, and per- 
sisted in coming forward with relatively little change 
until the twenty-seventh day; and even then the old 
conditioning held for most of the fish most of the 
time. Many individuals persisted in coming forward 
through the gate for a long time after they ceased 
biting or even swimming toward the acid-treated 

When a group of fish are reacting together, if a 
given individual moves forward through the gate to 
the feeding space, others may follow because of a 
group cohesion. It is obvious that if a fish is isolated 
and moves forward, the faster reaction cannot affect 
the behavior of other isolated fish. 

With this in mind. Dr. Welty undertook a series 
of experiments in which there were two partitions 
in the aquarium, with one door opening forward 
and another door opening through the other parti- 
tion toward the rear of the aquarium (Figure 34). 


The fish were placed in the central space and those 
in half the tanks were trained to come forward as 
usual. In the other half, two selected fish were con- 
ditioned to come forward and two were similarly 

A ^ 


Fig. 34. The aquarium-maze used in training part of 
the fish to come forward and part to go to the rear to 
be fed. (From Welty.) 

trained to move to the rear compartment to be fed. 
The experiment was tried several times with gold- 
fish, the minnow, Fundulus, common at Woods Hole, 
and another marine minnow, Cyprinodon. For one 
reason or another, only one series in which the fish 
were comparable was successfully completed. The re- 
sults are shown in Figure 35. Generally speaking, 
the cohering groups of Cyprinodon learned more 
rapidly and reacted more steadily than the separat- 
ing groups. This, then, is one factor that is working, 


at least at times, in causing grouped fish to learn 
more rapidly in a simple aquarium-maze than iso- 
lated fishes under similar treatment. 

As the goldfish move forward in the usual divided 



TR»AL 5 10 J5 19 

Fig. 35. Cyprinodon learn to move in a body more 
readily than to split into two separate groups. (From 

aquarium there comes a time when one or more fish 
may be in front of the screen, and the others in the 
rear of this advance guard. It was obviously a part 
of the investigation to find the effect these more 
rapidly reacting fish had upon their fellows merely 
as a result of being in the forward chamber. Con- 
ceivably they may have served as a lure. Another pos- 



sibility is that a rapidly learning individual becomes 
a leader in the reaction of the whole group. 

Both of these possibilities were tested experi- 



• 9 






• #1 


■ u 


■ n 


■ \l 




- \_ 



4 5 


Fig. 36. Goldfish learn more readily if accompanied 
by a trained leader than if there is a fish in the proper 
position to act as lure. (From Welty.) 

mentally by Dr. Welty, with results which are sum- 
marized in Figure 36. Three sets of aquaria were 
established. In the control aquaria all the goldfish, 
of which there were four in each tank, were fish 
which had had no previous experience in these ex- 
periments. These were trained as usual. In another 
set, an untrained fish was kept in each forward com- 


partment as a lure and four untrained fish were 
placed in the rear compartment. These fish were 
trained as usual; the so-called lure-fish was fed after 
the first of the untrained lot came through the gate- 
way. In the final set of aquaria a trained fish was in- 
troduced along with the four untrained fish. When 
the light was admitted and the gate was raised this 
trained fish moved forward, came through the gate- 
way, and was fed immediately. The others followed. 
As the graphs show, after the first day there was lit- 
tle difference in the reactions given by the control 
fish and by those which had a lure-fish in front of 
the screen. The fish with a trained leader generally 
gave more rapid reactions than either of the others. 
There is always a temptation to make comparisons 
between the learning behavior of these laboratory 
animals and that of men. Direct comparisons should 
usually be avoided. However, in human terms, the 
goldfish reacted more rapidly in the presence of a 
trained leader which went through the whole be- 
havior process with them, than they did to the pres- 
ence of one of their kind as a lure-fish in the forward 
compartment, a sort of signpost to proper behavior. 
Evidently leaders working with these goldfish can in- 
fluence them more than fish which by their posi- 
tion merely show them where they can come. It 
seems fair to say that with these fish demonstration 



teaching is the most effective method yet discovered. 

Still another attempt was made to study group 

cohesion in these goldfish. For this purpose aquaria 

Fig. 37. An aquarium-maze arranged to test the 
power of observation of fish placed in the side compart- 
ment. (From Welty.) 

were arranged like those in Figure 37. At the side 
of the usual aquarium-maze a narrow runway was 
placed into which untrained goldfish were intro- 
duced. In half of the tanks the glass partition was 
clear and allowed the fish to see the reaction of those 



in the larger aquarium-maze. In the other half, the 
partition was of opaque glass, cutting off the view. 


t 2 

Fig. 38. Goldfish react more rapidly if allowed to watch 

others perform. (From Welty.) 

Trained fish were placed in the aquarium-maze 

and were run through their performance from ten 

to twenty times in different experiments. The same 


treatment was given the fish in the aquaria with 
opaque partitions and those with clear glass. The 
trained fish were then removed and those from the 
small side chamber were gently transferred to the 
larger side. An hour later they were given an ordi- 
nary test such as had been given to the trained fish. 
As is clearly shown by the graphs in Figure 38, the 
fish which had been able to watch the others react 
behaved decidedly more like trained fish than those 
which had not been able to see their fellows perform. 

As a final check, the whole test was repeated, ex- 
cept that no fish were placed in the larger side of 
the aquarium. Fifteen times each aquarium was 
lighted up, the door opened, and the experimenter 
stood ready to feed any imaginary fish that might 
come through. Then when those in the side passages 
were transferred, there was no essential difference in 
the behavior of the fish from the two types of 
aquaria, and the experimenter was free from any sug- 
gestion that he might have been signaling the fish. 

The results of these experiments suggest that there 
is such a thing as imitation among goldfish. Whether 
there is or not depends, as Dr. Welty rightly says, 
largely upon the definition given to the word imita- 
tion. These fish probably do imitate each other on 
a relatively simple instinctive level. The untrained 
fish that watched the reaction of their trained fel- 


lows through the clear glass became conditioned in 
two ways which were not open to the fish behind 
an opaque glass. In the first place they saw the fish 
move forward on the reception of a given stimulus, 
pass through the gate, receive food, and give no evi- 
dence of an avoiding or "fright" reaction. This prob- 
ably gave what might be called a certain reassurance. 
Secondly, they showed group cohesion, and moved 
forward with the reacting fishes; at times they were 
even seen to move forward in advance of the fishes 
on the maze side of the aquarium. 

When transferred to the aquarium-maze and given 
the releasing stimulus of an increase in light, accom- 
panied by the opening of the gate, both types of 
previous experience probably played a role in pro- 
ducing a faster reaction. Fish behind the opaque 
glass could have neither of these helpful experi- 
ences. When their narrow aquarium was flooded 
with light they ordinarily moved back to the far end 
and remained there. There was nothing to train 
them to overcome this normally negative reaction. 
So reviewed, it must be said that this behavior has 
some points of resemblance to what is called imita- 
tion in other animals. 

There is also an element of imitation in the 
greater food consumption of grouped fishes. One fish 
sees another pursue, attack and consume a bit of 


food and its own feeding mechanism is set off as a 
result of this visual experience, even though its own 
hunger might not have been sufficient to stimulate 
feeding behavior. It is difficult to say to what ex- 
tent such behavior is an expression of competition 
as contrasted with unconscious co-operation. The 
two types of motivation overlap here and elsewhere. 

The evidence which we have been considering 
furthers our understanding of the fundamental na- 
ture of group activities among many animals, some 
of which are not usually regarded as being truly so- 
cial. The whole emphasis of this chapter has been 
laid upon facilitation as the result of greater num- 
bers being present. This kind of social facilitation 
has been described for such diverse processes as breed- 
ing behavior, eating, working and learning. 

Added numbers do not always facilitate these ac- 
tivities, as was shown by the analyses of the effect 
of numbers upon the rate of learning. With some 
animals, for example men and goldfish, under cer- 
tain situations, learning is more rapid with several 
present; but with others, such as parrakeets and mud- 
minnows, under the conditions tested, increased num- 
bers lead to a lower rate of learning. It seems that 
no all-inclusive positive statement can as yet be made 
in this field. One can, however, make the affirmation 
that in the general realm here considered the pres- 


ence of additional numbers by no means always re- 
tards, and is frequently stimulating. As before with 
regard to other processes, we find that in certain 
cases there are ill effects of undercrowding as well 
as ill effects of overcrowding. Without careful ex- 
perimental exploration, we cannot predict which 
effect will emerge from a given situation. 

One other result comes from these studies which 
will help us to clarify evidence still to be presented, 
as well as to review that already given. We have 
come upon another measure of the existence of so- 
cial behavior. Reactions may be regarded as social 
in nature to the extent that they differ from those 
that would be given if the animals w^ere alone. Such 
differences are frequently quantitative, as they have 
been in the cases we have discussed, although quali- 
tative differences occur as a result of a change in 
the numbers present. 

From this point of view social behavior may have 
or may lack positive survival value. All that is nec- 
essary is that the behavior be different from that 
which would be given if the animal were solitary. 
In this sense all the animals whose behavior we have 
been discussing are social to a considerable degree; 
the more so, the greater the difference between their 
behavior when grouped and when isolated. 

When the behavior of such animals as cockroaches, 


fishes, birds and rats shows evidence of distinct modi- 
fication as a result of more than one being present, 
we have another suggestion that there exists a 
broad substratum of partially social behavior. There 
are many indications that this extends through the 
whole animal kingdom. From such a substratum, 
given suitable conditions, societies emerge now and 
again as they have among ants and men. At these 
higher social levels, as is to be expected, the type 
of behavior shown under many conditions is related 
even more closely to the number of animals present 
than with less social cockroaches and fish. 


Group Organization 

WE ALL know that human society is more or less 
closely organized. Sometimes, as in military circles, 
some business organizations, and certain universities, 
there is a line organization which extends in a defi- 
nite order, step by step from the highest official to 
the lowest rank. Frequently, however, the organiza- 
tion is more complex, intricate and temporary. 

We have known for some time, too, that in herds 
of the larger mammals, where one can distinguish 
different individuals, the group may be organized to 
some extent with a dominant leader and frequently 
with sub-leaders that stand out above the common 
run of the herd. (16) 

Despite this knowledge we have found with sur- 
prise that other animal groups, a flock of birds for 
example, in which the different birds are indistin- 
guishable to the human eye, also are organized into 
a social hierarchy, frequently with a well-recognized 
social order which runs through the entire flock. 
The situation that has been revealed in these flocks 



of birds is amusing, interesting and important 
enough to warrant more attention than it is receiv- 
ing at present. 

Studies of the sort I am going to describe were 
initiated by a Norwegian named Schjelderup- 
Ebbe. (108) They were made possible by the use 
of colored leg bands and other markings by which 
the different individuals could be recognized by a 
human observer. Apparently the birds themselves 
knew the individual members of the flock without 
such artificial aids. 

Not because it is the most important work on the 
subject, but because I can best vouch for it in de- 
tail and in general, I shall present certain analyses 
of group organizations that have been made in our 
own laboratory. 

The organization of flocks of chickens is fairly 
firmly fixed. This is particularly the case with hens. 
The social order is indicated by the giving and re- 
ceiving of pecks, or by reaction to threats of peck- 
ing; and hence the social hierarchy among birds is 
frequently referred to as the peck-order. 

When two chickens meet for the first time there 
is either a fight or one gives way without fighting. 
If one of the two is immature while the other is 
fully developed, the older bird usually dominates. 
Thereafter when these two meet the one which has 


acquired the peck-right, that is, the right to peck 
another without being pecked in return, exercises it 
except in the event of a successful revolt which, with 
chickens, rarely occurs. 

The intensity of pair contact-reactions varies 
greatly. A superior may peck a subordinate severely, 
or lightly, or it may only threaten to do so. It usually 
turns its head, points its bill toward the subordinate 
and takes a few steps in that direction. It may then 
give a low deep characteristic sound which fre- 
quently accompanies an actual peck, and stretch its 
neck up and out without the resulting peck which 
it seems just ready to administer. 

The peck, when actually delivered, may be light, 
heavy, or slashing. These vigorous pecks may be 
painful even to man, as anyone can testify who has 
tried to take a setting hen off her nest; and particu- 
larly painful if repeated in the same spot. The peck- 
ing bird may draw blood from the comb or may 
pull feathers from the neck of the pecked fowl. The 
peck is frequently aimed at the comb or the top of 
the head; often it is not received with full force, for 
the pecked bird dodges. Less often the peck is di- 
rected toward back or shoulders. 

The severity of a peck which lands as aimed is 
illustrated by a recent observation in one of our 
small flocks. One bird received a vicious peck di- 



rectly on the top of its head; it walked backward 
two or three feet, staggered and fell, arose and again 
walked backward in a blind course that took it into 
the bird that had given the original peck. By that 

RW pecks 

all 12 

A, BG, BB, M, Y, YY, BG^, 



GY, RY, RR. 

RR pecks 


A, BG, BB, M, Y, YY, BG^, 



GY, RY. 

RY pecks 


A, BG, BB, M, Y, YY, BG2, 




GY pecks 


A, BG, BB, M, Y, YY, BG^, 



R pecks 


A, BG, BB, M, Y, YY, BG^, 


GR pecks 


A, BG, BB, M, Y, YY, BG2. 

BG2 pecks 


A, BG, BB, M, Y, YY. 

YY pecks 


A, BG, BB, M. 

M pecks 


A, BG, BB, Y. 

Y pecks 


A, BG, BB, YY. 

BB pecks 


A, BG. 

BG pecks 



A pecks 

Fig. 39. Flocks of hens are organized into a definite 
social hierarchy. 

time the aggressor had turned to eating and paid no 
attention to this chance contact. 

As a result of patient watching of pecks received 
and delivered, it is possible to find, with a high de- 
gree of accuracy, the social status of birds in a rela- 
tively small flock. (80) The organization of one such 
flock of brown leghorn pullets is shown in Figure 39. 
This peck-order was determined after sixty days of 
observation. As shown by the chart, there was a 
regular line organization down to the eighth bird. 


Then a triangle was encountered in which M pecked 
Y, Y pecked YY and YY pecked M; and each of these 
had the peck-right over the remaining members of 
the flock. 

Such irregularities are by no means uncommon 
even in well-established flocks. A hen which is 
otherwise the alpha bird in the pen may be pecked 
with impunity by some low-ranking member, al- 
though the latter is in turn pecked by many birds 
over which the alpha hen has a clearly established 
social superiority. This inconsistency may result 
from the low-ranking bird having first met the 
alpha bird on one of its off days, gained the advan- 
tage in the first combat and managed to keep it 
thereafter with the aid of the psychological domi- 
nance thus established. 

Similar social hierarchies exist also among flocks 
of male birds. One flock of cockerels, which we 
studied for seventy days, demonstrated the social 
order shown in Figure 40 in which there are six 
triangle situations that run through all the upper 
part of the social scale, but are especially evident in 
the middle ranks where B is involved in four of 

Cockerels are more pugnacious than pullets, even 
when they are kept, as these were, on a diet which 
somewhat restricts the tendency to fight. There were 


more revolts and these were more likely to be suc- 
cessful. For example, in this flock of cockerels, the 
four birds lettered in bold-faced type in Figure 40 
showed reversals, and with some the social rank had 

BW pecks 9: W, BY, G, RY, B, BG, Y, R, GY. 

BR pecks 8: W, BY, G, RY, BG, Y, R BW. 

GY pecks 8: W, BY, G, RY, B, BG, Y, * BR. 

R pecks 7: W, BY, G, RY, B, BG, GY. 

Y pecks 6: W, BY, G, RY, BG, R. 

GB pecks 5: W, BY, G, RY, B. 

B pecks 4: W, G, RY, Y. 

RY pecks 3: W, BY, G. 

G pecks 2: W, BY. 

BY pecks 2: W, B. 

W pecks o. 

In this order there are six triangle situations as follows: 


/ \ /\ A /\ A /\ 

B^^ BR Y-e— GY Y-^ B Y^h- B BY^RV B-^BY 

Fig. 40. Cockerels also have a social organization which 
is, however, somewhat more confused than with hens. 

not been finally determined even after seventy days 
of observations. Thus BY was observed to peck G 
on six occasions, while G pecked BY eight times. 
Ideally, in work of this kind, the birds should be 
kept under observation throughout their waking 
hours in order that we may have the full history of 
their behavior. Such prolonged watching is imprac- 
ticable, particularly since during much of the day 
there is little pecking. Actually, observations were 


restricted to the time near feeding, when the birds 
were most likely to fight. Taken together with the 
greater number of triangles, the reversals indicate a 
less stable social order among these male birds than 
among their sisters. 

For a time there was no completely dominant bird 
among the cockerels. BW, which stood highest in 
general, was pecked by BR, which ranked otherwise 
just below him. One day BR and Y started to fight, 
as they had done many times before, with BR win- 
ning. This time Y struck through to the eye, which 
closed as a result, and BR retreated. The injury was 
such that the tender-hearted observer thought that 
BR needed special treatment, and removed him to 
a hospital pen. The eye healed, and two weeks later 
the recovered bird was returned to the flock which 
he had almost dominated. In these two weeks of 
absence he had lost his social status entirely, and 
was pecked even by W, which had not been seen 
before to peck a fellow cockerel. The reason for his 
loss of position is not clear. He had been severely 
injured, he had lost a fight to an inferior, and he 
had been absent from the flock for fourteen days. 
For one or all of these reasons he had lost caste so 
completely that five days later he had to be removed 
from the flock, literally to save his life. 

During the five days that BR was again with the 


flock, he avoided contacts with others as much as 
possible, and spent a great deal of his time crowding 
under a low shelf on which the water dish was kept. 
In our experience, the lowest ranking chicken in a 
flock tends to avoid social contacts as BR did after 
his fall from superior position. Frequently the low- 
ranking birds show many objective signs of fear. 
They spend time in out-of-the-way places, feed after 
others have fed, and make their way around cau- 
tiously, apparently with an eye out to avoid con- 
tacts. The lowest ranking birds may appear lean, and 
their plumage is somewhat more rumpled because 
they have less time to arrange it. Dominant birds, 
on the other hand, are characterized by a complete 
absence of signs of fear or of any attempt to avoid 
birds of lower ranks. Some birds, usually those high 
in the peck-order but not at the top of it, show few 
avoiding reactions to their superiors, and, when 
pecked, apparently take it lightly and pass on. 

Chickens show some other interesting reactions 
which are related to their position in the social 
hierarchy to which they belong. Professor Murchi- 
son, a psychologist at Clark University, has reported 
studies on the behavior of a flock of six cocks and 
five pullets. (83) In one series of experiments pair 
after pair of the cocks were selected at random and 
placed at either end of a narrow runway behind 


glass doors which allowed them full sight of each 
other. When the glass doors were opened the cocks 
ran toward each other. The point of meeting was 
proportional to the relative position of the two in 
the social scale, for the more dominant bird traveled 
farther than the subordinate one. 

In another experiment two cocks were placed in 
small wire cages in which they were plainly visible, 
and these cages were set in an enclosure about six 
feet apart. If a third male from the flock were intro- 
duced into the pen the free bird would go toward 
the caged cock which was relatively lower in the 
social scale. In this it behaved exactly opposite to 
the females which were members of the same flock 
and "acquainted" with both roosters. A hen released 
under similar conditions is said to make her way 
toward the cock that has the higher social position. 

In our studies we have usually found that the 
birds higher in the social order had more social 
contacts than those that were at the bottom of the 
peck-order. The correlation is not always exact, but 
to date we have found few exceptions to the rule 
that the bird lowest in the peck-order has the fewest 
contacts. A quantitative difference, closely associated 
with social rank, may be found in the number of 
pecks delivered when there is no difference in the 
total contacts among the upper birds. In a recent 


Study (9) in which four pens were under observation 
with five or six pullets in each, out of 4,400 pecks 
the ranking birds gave 1,800, the second in the lists 
gave 1,092, and so on in regularly declining num- 
bers until those next to the bottom gave 136 and 
the birds that were lowest in their respective flocks 
gave none at all. 

Murchison has reported a variation of this general 
rule. In studying the sexual behavior of his birds, 
of the three cocks that gave the mating reaction the 
number of treadings stood in direct relation to social 
position, with the ranking cock treading pullets 
most frequently. Interestingly enough, the top pullet 
was also the bird which mated most frequently, and 
the number of matings of the remaining females 
was in direct proportion to their social position. 
This appears to be a special case of the general rule 
that birds high in the peck-order have more social 
contacts than those that are low in social rank. 

These are some of the known relationships exist- 
ing among birds that have a relatively fixed group 
organization. Schjelderup-Ebbe, (109, 110) who has 
made observations on over fifty species of birds, in- 
cluding, besides the common chicken, a sparrow, 
various ducks, geese, pheasants, cockatoos, parrots, 
and the common caged canary, is convinced that des- 
potism is one of the major biological principles; 


that whenever two birds are together invariably one 
is despot and the other subservient and both know 
it. He has said, "Despotism is the basic idea of the 
world, indissolubly bound up with all life and exist- 
ence. On it rests the meaning of the struggle for 
existence." He applies this principle to interactions 
of men and of other animals and even to lifeless 
things. He says: "There is nothing that does not 
have a despot . . . usually a great number of des- 
pots. The storm is despot over the water; the light- 
ning over the rock; water over the stone which it 
dissolves"; and he cites with approval the old Ger- 
man proverb that God is despot over the Devil. 

This poetry of Schjelderup-Ebbe's is striking, but 
does it rightly interpret the facts? We have spent a 
considerable amount of time at Chicago, investi- 
gating the social order of various birds. Messrs. 
Masure, Shoemaker, Collias and Kellogg and Miss 
Bennett have been particularly active iii this work. 
We have not yet studied as many varieties of birds 
as Schjelderup-Ebbe, and we have no experience to 
report about the relation between God and the 
Devil. Of the birds we have studied, only the flocks 
of white-throated sparrows approach the common 
chickens in the fixity of their social hierarchies, and 
they do not equal it. The common pigeon, the ring 
dove, the common canary and parrakeets show a less 


rigid type of social organization which I can illus- 
trate by explaining the situation as we have found 
it among common pigeons. (80) 

The observations were made on a group of four- 
teen white king pigeons, half of which were male 
and half female. Their social order was observed in 
sex-segregated flocks until, after a month, it seemed 
to be fairly stable; then the flocks were combined, 
and after a month during which five of the seven 
possible pairs mated, the sexes were again segregated 
for twenty-eight days of further study. The results 
are essentially similar both for the males and the 
females for the period when the sexes were separate, 
so that I shall follow only the reactions of the fe- 
male flock. The essential facts can be described with 
the aid of the diagrams in Figure 41. These show 
the social interactions between the females lowest in 
the social order. 

Let us examine Chart A with some care. This 
charts the relationships of the five birds that were 
lowest in the pre-mating flock. All these were domi- 
nated in the main by BY and BB. The figures show 
that BR was seen to peck GW ten times and was 
pecked by GW, and retreated from her nine times. 
GW pecked BW thirteen times, but lost in four 
encounters. BR won ten and lost seven of its ob- 
served contacts with BW, which won thirteen and 



lost ten with RY. RY in turn was practically even 
(eight to seven) with BR and slightly ahead in its 
relations with GW and RW. I do not intend to sug- 











13 UO 


Fig. 41. In flocks of pigeons the organization is one 
of peck-dominance rather than of peck-right. The 
pigeons highest in the social order are omitted from 
these diagrams. A, the pre-mating flock; B, the entire 
period of observation; C, the post-mating flock. 

gest that most of these differences are important; in 
fact that is the point. With flocks which are organ- 
ized as are these pigeons, it frequently becomes diffi- 
cult to decide which bird stands higher in the social 

It is important to note that in none of these cases, 


in fact in only one of all the different reaction pairs 
whose behavior is summarized in these charts, was 
there an absolute dominance of one bird by the 
other, and then only two contact reactions were 
seen. When all contacts throughout the whole period 
of observation are considered, there was at least one 
time for each of the contact pairs when the bird 
which usually lost out dominated the contact reac- 

In Chart B, which shows all the reactions during 
pre- and post-mating, and in C which records the 
contacts for the post-mating season only, the four 
birds represented by the diagrams were dominated 
by three others, RY, BY and BB. It is worth empha- 
sizing that with these birds an absolute despotism 
was not established. Even RY, which more than any 
other bird dominated the post-mating flock, lost con- 
tact reactions to each of the others except to RW, 
which was lowest of all. While it was winning 329 
reactions it lost 58, and each of the other females, 
RW excepted, dominated it at least three times in 
the post-mating observations. 

The picture that emerges is one of a flock which 
is organized into a social hierarchy, but one which 
is not so hard and fast as that found with chickens. 
In the long run one becomes fairly sure which bird 
in each of the groups will dominate in the larger 


number of their contacts, but the result of the next 
meeting between two individuals is not to be known 
with certainty until it has taken place. Within the 
same hour and even within a few minutes reversals 
in dominance may take place without anything un- 
usual in the circumstances. 

Putting the matter somewhat facetiously, chickens 
appear to have developed the sort of "line organiza- 
tion" characteristic of a military system or a fascist 
state, while these pigeons, together with the ring 
doves, canaries and parrakeets, are more democratic. 
The social hierarchy among chickens is based on 
an almost absolute peck-right which smacks strongly 
of the despotism of which Schjelderup-Ebbe writes, 
while these other birds have an organization based 
on peck-dominance rather than on absolute peck- 

With such birds social position is not fixed once 
and for all. Consider the case of RY among pigeons. 
When results were first thrown together at the end 
of two weeks of observation, RY was at the bottom 
of the flock, a position which it retained for twelve 
more days. Then something began to happen. What 
it was, I wish I knew. RY began to go up in her 
social world. After six days she. ranked a shaky third, 
clearly dominated on the average by BY and BB. 

Then the pigeons were allowed to mate. During 


the mating period BY, which was top bird in the 
pre-mating flock of females, and RY did not pair off 
with any of the males. Again I do not know why. 
After the experiment was finished RY was carefully 
autopsied and we could find no evidence of any- 
thing physically abnormal. When the sexes were 
again segregated RY was the top-ranking bird among 
her fellow females, and remained so. She was seen 
to have loi contacts with BY, the former alpha bird, 
and to win 83 of them; she had 77 observed contacts 
with BB, which had formerly been second from the 
top, and defeated her 53 times. In the pre-mating 
period RY lost two combats for each that she won; 
in the post-mating flock she won five contacts for 
each that was lost. 

This raises in a rather dramatic fashion questions 
as to what qualities make for a dominant bird. This 
problem is not yet solved. With these birds, social 
rank is in part a matter of seniority. Mature chickens 
usually dominate immature ones and maintain their 
dominance long after the former youngsters have 
become fully mature and possibly physically able to 
displace the senior members. This is good evidence 
that memory of former defeats plays a role in main- 
taining the social order once it is established. When 
chickens strange to each other are put together for 
the first time dominance usually goes to the bird 


with superior fighting or bluffing ability. Maturity, 
strength, courage, pugnacity and health, all seem 
essential qualities making for dominance among 
chickens. Luck of combat also seems to play a part 
when one considers the numerous triangle situations 
that have been discovered. Since cockerels have cer- 
tain of these qualities more than pullets, a male 
bird, if present, dominates a flock of hens. 

There seems to be little if any correlation between 
greater weight and position in the peck-order. The 
location of the combat seems to be important. 
Schjelderup-Ebbe found that chickens in their home 
yard win more combats than strangers to that yard; 
and Mr. Shoemaker has reported that, with canaries, 
each bird becomes dominant in the region near its 
nest. (113) We found some years ago that with 
pigeons one might be dominant on the ground 
about the feed pan and another have first rank at 
the entrance to the roosts. (80) 

With chickens, as I have said, the larger, stronger, 
more pugnacious males usually dominate the fe- 
males. This is said to be generally true in species 
in which the male is larger or more showy than the 
female. With the parrakeets, (11) whose social order 
in many ways resembles that of pigeons, the females 
are dominant over the males except in the breeding 
season. While breeding and nesting are in progress 


positions are reversed, and a previously hen-pecked 
male may drive his usually dominant mate back 
onto the nest when she attempts to leave it. The 
sexes in these parrakeets can be told apart only by 
slight differences in color. 

When hens are giving the brooding reaction or 
are caring for small chickens, they become less sub- 
missive to other hens. Some of the other birds, whose 
social ranking has been investigated, move up and 
down in the social scale according to the phase of 
the breeding and nesting cycle which they are in at 
the time. 

It has been reported that with hens those high in 
the peck-order have a higher IQ than their more 
lowly placed flock mates. (72) The IQ was measured 
in this case by placing grains of corn out on the 
floor with every other grain securely fastened down, 
and finding the speed and accuracy with which the 
fowls would learn to peck at the loose grains only. 

We have had as yet only the most casual personal 
contact with this problem so far as chickens are con- 
cerned. With the parrakeets, Masure and I could 
find no evidence of a positive correlation between 
any aspect of ability to learn a maze and social rank. 

From this summary it is evident that in spite of 
a great deal of study we do not know all the factors 
which determine the position of birds in their social 


order. There is some suggestion from the effect of 
broodiness in hens and from observations on the 
nesting cycle in canaries that there may be elements 
of control by hormones. This lead is being investi- 
gated actively at present, but I have no definite re- 
sults to report. (6) 

Some of the complications in determining the fac- 
tors that make for dominance are shown by the pre- 
liminary summary which Mr. Shoemaker has given 
me of his studies on the social hierarchy in canaries. 
The space available for the caged flock is a matter 
of importance. When confined in relatively small 
space, the social order becomes more simple and 
definite and there is no complication over the ques- 
tion of territorial rights. With more space, as for 
example in a large flight cage, individual territories 
tend to become established in which the particular 
bird is supreme even though it ranks low in the 
neutral ground around the bath bowls, the feeding 
places, or regions where nesting material is stored. 

When canaries are allowed to mate and small 
nesting cages are supplied around the walls of the 
flight cage, each individual male is master in its own 
nest cage and controls more or less territory around 
the cage entrance. Under these conditions even the 
birds lowest in the social order dominate in some 
restricted space about their nest. 


In general these canaries show more pecking 
among the males than among the females, and dur- 
ing the nesting period the female does little to de- 
fend the nest territory; that is the work of her mate. 
In this home territory the social dominance of the 
male over his fellow males is not steady but varies 
with different phases of the breeding cycle. During 
the processes of nest-building, egg-laying and incu- 
bation, the male tends to become more dominant. 
This is shown by an increase in the size of the ter- 
ritory about the nest which he dominates, and by 
the fact that when on neutral territory he tends to 
win more of his pair contacts. During the rest of 
the cycle the male tends to lose dominance as meas- 
ured by both these criteria. 

It is worth noting that in the course of these pul- 
sations in dominance the male may not actually 
move up in the social scale as determined by the 
number of birds which he fully dominates. He may 
win more of his individual pair contacts without 
actually oversetting the usual trend. The same bird 
may show fluctuations in dominance during the day. 
Thus one male regularly dominated less territory of 
an evening than he did in the morning. This may 
well be a matter of stamina. 

In some cases the relation between the sexes in 
these canaries hinges on another complication. For 


example, a female, 15, mated with a male, 55, which 
stood about midway in the social order among the 
males: 55 dominated the other females and all the 
other males dominated over 15. However, of thirty- 
three observed contacts between 15 and 55, the male 
lost all but one! The male parrakeet will drive back 
on her nest the female who has left it, but 55, like 
other male canaries, coaxed his mate back to her nest 
with offers of food. 

Until studies are further advanced, we cannot be 
sure how many of these complications which Mr. 
Shoemaker has recorded for the canaries are found 
elsewhere even among birds. It seems reasonable to 
suppose, however, that the social hierarchy is rarely 
as simple in its organization as a mere listing of the 
social ranking seems to indicate. 

With all these birds, high rank in the social order 
of the flock means much greater freedom of action, 
more ready access to food and a generally less 
strained style of living. It is hard to say whether in 
nature it means more than this, although it seems 
probable that in times of food shortage, or other 
phases of environmental stress, the ranking birds 
who have the first opportunity at food might readily 
fare better than those low in t,he social scale. Fortu- 
nately, enough observations have been made in na- 
ture so we know that with some species the peck- 


order, which has been most studied in restricted 
cages and pens, does occur in the wild. 

The alpha bird in a penned flock of chickens does 
not necessarily lead in foraging expeditions when 
the flock has more space. Fischel, a German, reports 
that when hens of known peck-order are released 
to forage in an orchard the dominant and near- 
dominant birds may or may not be at the apex of 
the foraging flock. (46) Usually the leadership 
changes from time to time and moreover the lead- 
ing bird seems always more or less dependent upon 
her followers. If she gets too far out ahead the leader 
turns back and rejoins the flock or waits for them 
to catch up. Similar hesitation by the leader when 
it has advanced some distance in front of its fol- 
lowers has been observed among other animals, no- 
tably among ants and men. 

This problem of leadership among birds is related 
to, but not identical with, position in the social 
order. There are many aspects of the problem into 
which we cannot go at present, pending a closer and 
more revealing study than has appeared as yet of 
the qualities that make for leadership. 

With some herds or hordes of mammals leader- 
ship rests with an old and experienced female. (16) 
In such herds the females and young frequently 
make up the more stable part of the social group. 


to which males attach themselves during the mating 
season. With other mammals the male is the leader, 
and sometimes a jealous one, that drives other males 
out of the herd; although in some cases several males 
are tolerated. (3) 

Leadership does not always go to the faster or 
stronger animal; in fact, the position of being out 
in front of the flock may not mean real leadership. 
An interesting example of such pseudo-leadership 
has been recorded for a mixed group of shore birds 
observed by Mr. Nichols of the American Museum 
of Natural History. (85) 

He found a mixed flock of such birds which was 
composed of two young dowitchers, with a dozen 
black-bellied plovers and a single golden plover. 
Under these conditions certain of the birds could 
readily be distinguished from the others. When the 
flock was flushed, the flight of the golden plover was 
comparatively rapid and it was soon ahead of all of 
the rest. The dowitchers were slow and tended to 
fall behind, and when this happened the black- 
bellied plover wheeled. This affected both the ap- 
parent leader, the golden plover, and the lagging 
dowitchers. The former, finding itself alone with- 
out followers, rose above the. flock, took the new 
direction and dived down with a few swift wing 
beats, again the apparent leader of them all. The 


slower dowitchers took the chord of the arc made 
by the wheeling flock and so caught up with and 
again became an integral part of the flying group. 
Soon again the slow dowitchers lagged and the whole 
performance was repeated. 

These observations do not reveal the stimulus 
which releases the wheeling mechanism of the main 
flock. The simplest explanation, that the leader, find- 
ing himself out alone in front, starts to turn and 
so gives a stimulus to the keen-sighted remainder so 
that they also shift direction almost instantaneously, 
does not hold in this mixed flock, for the observa- 
tions indicate clearly that the apparent leader, the 
golden plover, was following along in front of the 
main flock as much as the slow dowitchers were fol- 
lowing along behind it. 

Neither does this simple-leadership sort of ex- 
planation fit the facts as observed among wheeling 
flocks of other shore birds or of pigeons. In such 
flocks the stimulus to turn frequently seems to 
originate in one of the flanks, and spreads from that 
point rapidly through the flock. Here again the ap- 
parent leaders may not be the actual ones. It is pos- 
sible, though we are not yet sure of it, that in such 
flocks made of birds which we cannot tell apart, the 
faster individuals also may dive through the flock 


to the foremost position, taking their direction from 
the whole flock. 

However the signal for turning, originates, the 
wheeling takes place so rapidly that mythical ex- 
planations are still being advanced. I have a small 
book written on the subject by an English author, 
called, Thought-transference (or What) in Birds? 
(ill) The title correctly summarizes the contents of 
the book. 

I would not have you conclude from my repeated 
emphasis on the absence of definite leadership in 
these flocks of birds, and on the presence of a 
pseudo-leadership when the flock is really determin- 
ing the direction that is taken by the bird in front, 
that there is no real leadership among other animals 
and among men. And I must make it clear that here 
I am speaking of real leadership and not of a peck- 
order, which, as is true with social position in human 
society, does not imply leadership at all. Such a po- 
sition could not be successfully maintained by a per- 
son trained in science rather than in dialectics. But 
apparently, at least among so-called lower animals, 
the leader is frequently as dependent on his fol- 
lowers as they are on him, and sometimes even more 
so. A similar situation occurs in human affairs often 
enough and under such a variety of situations that 
the relationship deserves more careful consideration 


than it usually receives when problems of leadership 
are discussed. 

While those of us who have been engaged in these 
studies have probably never been wholly unaware 
of the possibility of amusing cross-references to man, 
I must insist that our motivation has not been that 
of making an oblique attack upon human social re- 
lations. Rather, we have found problems concerned 
with the social organization of birds and other ani- 
mals interesting and important on their own ac- 

We have, of course, a feeling that different ani- 
mals have much in common in group psychology 
and in sociology, as well as in more distinctly physio- 
logical processes. It is the viewpoint of general 
physiology that we cannot understand the working 
and the possibilities of the human nervous system, 
for example, without study of the functioning of 
the nervous systems of many other kinds of animals. 
Similarly well-integrated information has been com- 
piled concerning general and comparative psychol- 
ogy. From the same point of view some of us have 
been trying to develop a general sociology, which 
even in its present imperfect state allows human 
social reactions to be viewed in part as the peculiar 
human development of social tendencies which also 
have their peculiar developments among insects. 


birds, fish, mice, and monkeys; that is, among social 
animals generally. 

Keeping this point of view, and with our back- 
ground of studies of social organization, it is worth 
while to turn for a short consideration of the actual 
application of similarly objective studies in certain 
human groups. I pass over the possibilities of study- 
ing the peck-order in women's clubs, faculty groups, 
families or churches, to call your attention to some 
studies that have recently been published dealing 
with the social interactions of the Dionne quin- 
tuplets, since these will serve to throw light on a 
number of interesting points. (25) 

In all questions of dominance in the group or of 
other forms of social inequality, we come immedi- 
ately and continually upon the question of the ex- 
tent to which these observed social differences are a 
matter of heredity and to what extent they follow 
differences in training or other environmental im- 
pacts. This is the old nature-nurture problem, other 
aspects of which have been discussed for years. 

Driven by many different kinds of evidence, biolo- 
gists have come to the conclusion that all men are 
not born equal. Applying this to social affairs we 
have the general assumption that many of the ob- 
served differences in social position are a result of 
the inherited differences depending on the vagaries 


of bi-parental inheritance and more remotely on 
mutations of one kind or another. 

Fortunately we have in the case of the Dionne 
quintuplets a natural experiment which deserves 
much attention. Detailed biological studies which 
appeared late in 1937 confirmed the general assump- 
tion that these much-discussed babies are an iden- 
tical set of sisters. Biologically this means that all 
of them have come from one ovum which was fer- 
tilized by one spermatozoan. Soon after fertilization 
the early cleavage cells separated and produced five 
embryos, each with identical heredity. I shall not 
give the details of the evidence on which this con- 
clusion is based. In addition to looking so much 
alike that only their regular attendants can tell them 
apart with any degree of sureness, there are simi- 
larities in finger and palm prints, in toe and sole 
prints, and in other anatomic details which point 
conclusively toward a common identical heredity. 

A group of investigators from the University of 
Toronto have been studying the social reactions of 
the quintuplets and have reported observations from 
the twelfth to the thirty-sixth month of their age. 
At first the children were placed together in a play 
pen by pairs to observe their interactions; from the 
twenty-second to the thirty-sixth month they were 
observed as a group. 


The available records do not allow an exact com- 
parison with the peck-order I have described for 
various birds. The observers were interested in re- 
cording and analyzing the following bits of behavior: 

1. Total contact reactions. 

2. Reactions of one child toward another, which 
they call to reactions. A to reaction by one child 
will be a jrom reaction for the child receiving the 

3. Whether the reactions are initiated or are re- 
sponse bits of behavior. An illustration will help to 
make this clear. If A pushes Y, it is regarded as an 
initiated to reaction by A, while Y is credited with 
a from reaction. If Y pushes back, then this is a re- 
sponse to reaction for Y and a from reaction for A. 

4. They also record which child watched which 

I shall not use all these distinctions for my points 
can be made accurately with only part of them. 

As shown in Figure 42, certain reactions are 
summarized in the top row for the entire period 
from the twenty-second to the thirty-sixth month, 
and in the lower row the same reactions for the last 
four months of the study, from the thirty-second to 
the thirty-sixth month. The left-hand diagrams give 
the total contact reactions during these respective 
periods. The center diagrams show the total to reac- 


tions and those on the right give the initiated to 

Let us examine the upper left figure. A had a 

AGE: 22-36 MONTHS 















































100 84 74 58 54 




2 3 15 4 

100 50 70 61 53 


74 68 60 53 «— PER CENT 
















100 90 62 52 40 

100 85 61 56 37 
AGE: 32-36 M0NTH5 

67 62 37 34 <— P£R CENT 

Fig. 42. The Dionne quintuplets also show evidence of 
a social organization among themselves. 

total of 740 observed contacts directed to and re- 
ceived from her sisters. This is taken as lOO per cent. 
C had similarly 622 contacts, which were 84 per 
cent of A's, and so on, with Y third, M fourth, and 
E fifth. The order in total number of contacts then 


is A, C, Y, M, E. This same order holds for total to 
contacts and for both total and to contacts in the 
thirty-two to thirty-six months' period. The diagrams 
on the right show that A initiated the most to con- 
tacts, and that C was next. Beyond that the order 
varies. For the whole period of observation (upper 
row) it is M, E, Y, and for the last period (lower 
row) it stands as M, Y, E. 

The other available data do not always give this 
same order, but enough has been presented to show 
that, among these children identical in heredity and 
almost so in post-natal environment, there are social 
differences which can be recognized by the behavior 
of the children toward each other. 

As the figures giving mental rank indicate, the 
correlation with intelligence is by no means perfect. 
Neither is the correlation with size. Y, the largest, 
and said in some ways to be the most mature of the 
five, ranks in the tests shown here from third to 
fifth. And while M, the smallest, ranks low, she is 
not the lowest, and other data show that in the per- 
centage of her contacts which were self-initiated to 
reactions she ranks first of all these sisters. 

These observed differences raise an interesting 
question: If heredity has been the same and the 
environment constant, how did the differences creep 
in? It is possible that there are unobserved, unrec- 


ognized differences either in the handling of the 
children, in their early contacts with each other, or 
in their impacts with their physical environment 
which may have been cumulative enough to pro- 
duce these social differences. It is also possible, as 
Professor H. H. Newman suggests, that the differ- 
ences are environmental after all. We must remem- 
ber that from the standpoint of A, C, Y, M, and E, 
their environmental relations began long before 
birth, and though the care given them since birth 
may have been practically identical in each case, it 
may not have been possible to erase environmental 
conditions impressed upon them during their seven 
critical months of intra-uterine life. 

Whatever the reason, we have come to an inter- 
esting, and, I think, important conclusion, which is 
that animals with exactly the same heredity may 
still develop, even at an early age, graded social dif- 
ferences showing that one is not exactly equal to 
the other. We have indications that the same prin- 
ciple holds among birds, but even if present indica- 
tions are finally borne out, the experiment will not 
be as elegant, in the strictly scientific sense, as are 
these observations on the Dionne quintuplets. 

Finally, by way of review, there exists among 
flocks of birds, even though they may be identical 
to the human eye, a graded series of reactions within 


the flock which allow observers to rank the birds in 
the order of their social dominance. This social 
order may be relatively hard and fast, as with hens, 
or more loosely organized on a give-and-take basis 
among pigeons and canaries. The factors under- 
lying the social order in these birds are complicated 
and include such personal traits as age, pugnacity, 
sex in general and the reproductive cycle in par- 
ticular, as well as such environmental factors as size 
of available space and the possibilities of establish- 
ing special territories. High position in the social 
order does not necessarily coincide with group lead- 
ership, although at times it does. The survival value 
of high position in the social hierarchy has not been 
demonstrated, but there are many reasons for sus- 
pecting that it may be felt in times of famine or 
during other periods of environmental stress. 

The problems related to leadership, although 
mentioned, were not discussed exhaustively. Em- 
phasis was laid on the importance to the leader of 
his followers, and on the existence of a pseudo- 
leadership in which the animal in front is taking 
direction from his apparent followers. 

With the Dionne quintuplets it was demonstrated 
that social differences exist even with children that 
have identical heredity, and a theory of environ- 
mental differences was favored as an explanation. 


In conclusion, the social organization observed in 
birds and other animals reminds one almost con- 
stantly of certain types of human social situations. 
The dominance-subordination relations of people 
are at times readily observed; at other times they 
are obscured by other social responses. When present 
in man, patterns of domination may be expressed in 
many more ways than in birds or mice. It may well 
be that the social hierarchies of chickens, canaries 
and men have much in common. Without taking 
the comparison too seriously, the fact that chickens, 
for example, have a relatively simple system of des- 
potism may help explain, though it does not justify, 
the appearance of a similar social organization in 
man. Other types of social organization also exist 
among the other animals, and man need develop 
only that best suited to his unique situation. 


Some Human Implications 

WHILE WE have been engaged in trying to assay 
the relative importance of the principle of co-opera- 
tion among animals, we have given most of our time 
and attention to its manifestation among animals 
considered to be asocial or only partially social. In 
such animals it is an unconscious kind of mutualism, 
but its roots are deep and well established and its 
expression grows to be so spontaneous and normal 
that we are likely to overlook or forget it in the more 
striking exhibition of social co-operation among 
higher animals. Conscious co-operation is so com- 
paratively new in an animal world many millions 
of years old, that we may underrate its strength and 
importance if we are not reminded of its foundations 
in simple physiology and primitive instinct. 

When we attempt to apply to human behavior the 
same methods of analysis that we have used through- 
out toward other animal groups, we reach most in- 
teresting results when we select some phase of reac- 
tions of men in which integration has not developed 



much beyond that found in some of the semi- or 
quasi-social animal aggregations which we have been 
considering in the lower animals. 

Among the possible aspects of human behavior 
that meet this requirement and that lend themselves 
to biological analyses is the whole set of activities 
that center about the relations between nations. 
Even the most optimistic humanist will not main- 
tain that these are at present, or ever have been, on 
as high a social plane as that which characterizes 
many of the personal interactions of mankind, or 
those of the smaller social groupings of men. 

The most casual reading of recent events is con- 
vincing evidence that the modern international sys- 
tem is based on war. This final resort to violence 
has been regarded by many thoughtful people as in- 
evitable, man being what he is, that is, the product 
by natural selection of the results produced by the 
struggle for existence; for the ordinary thoughtful 
person is not aware that the tendency toward a strug- 
gle for existence is balanced and opposed by the 
strong influence of the co-operative urge. Because 
of this common attitude toward war, and because 
of its fundamental importance to our species, I pro- 
pose to cut through the shifting tangle of interna- 
tional policies down to the basic biological signifi- 
cance which it holds for us. 


In doing so I must recognize these two funda- 
mental principles, the struggle for existence and the 
necessity for co-operation, both of which, consciously 
or unconsciously, penetrate all nature; and I shall 
say now that we may find that these two principles 
are not always in direct opposition to each other; 
that there is evidence that these basic forces have 
acted together to shape the course of evolution, even 
the evolution of social relations among men and na- 
tions of men. 

If, in the past, we have not had facts on which 
to base rational conclusions about national problems, 
it cannot be said that we have not had powerful 
emotions to drive us into one attitude or the other. 
It is very difficult to keep an objective, unemotional 
attitude toward the complex subject of the biology 
of war. We may not agree in our placing of the 
emphasis, but I trust that when we disagree it will 
be on a healthy intellectual level. 

It is clear that we are entering a tricky field where, 
to a greater extent than usual, the evidence is not 
all in, and one in which much that we think we 
know is contradictory. No one can bring this prob- 
lem into the laboratory for careful testing. We must 
do the best we can with inforraation which is more 
incomplete and faulty than that on which we nor- 
mally base our biological discussions. The human 


importance of the subject justifies the risk. The pres- 
ent discussion will center about three main points: 

1. To what extent do the underlying biological 
relationships tend to bring about war? 

2. Is war biologically justified by the results pro- 

3. Can the basic principles of struggle and of co- 
operation work together in the international rela- 
tions of men? 

Many men are aggressive animals. The similari- 
ties between human social hierarchies and those of 
chickens and other animals emphasize similarities of 
the drive toward dominance in the species concerned. 
Our immediate question is: Does this human aggres- 
siveness mean that men have an inherent, instinctive 
drive toward war? The ideal way to attack this prob- 
lem would be to rear sizable groups of people free 
from contact with outside influence or social tradi- 
tion and see whether under these conditions they 
would instinctively engage in group combats in order 
to forward or defend group ambitions. 

Such objective procedure is out of the question, 
but an interesting subjective inquiry has been made. 
In 1935, American psychologists took a poll among 
themselves on the question as to whether they be- 
lieved that the tendency toward making war is an 
instinctive drive in man. Of those answering^, well 


over 90 per cent said that there is no proof that war 
is an innate behavior pattern. (129) Less than 10 per 
cent thought that war represents an instinctive re- 
action. I did not personally see this questionnaire 
but I am credibly informed that the question was 
stated fairly and did not suggest the type of answer 

This is a rather unexpected unanimity, and may 
be accounted for to a minor degree by the existence 
of one modern school of psychologists that doubt 
the possibility of instinctive action, particularly 
among men. I do not think they represent a large 
proportion of American psychologists but there may 
have been enough of them to have lifted the per- 
centage high. 

The opinion of the psychologists is supported by 
the independent judgment of one of the leading 
students of anthropology, Professor Malinow^ski, who 
said in his Harvard tercentennial lecture: (78) "All 
the wrangles as to the innate pacifism or aggressive- 
ness of primitive man are based on the use of words 
without definition. To label all brawling, squab- 
bling, dealing of black eye or broken jaw, war^ as is 
frequently done, simply leads to confusion. War can 
be defined as the use of organized force between 
two politically independent units, in the pursuit of 


tribal policy. War in this sense enters fairly late 
into the development of human societies." 

It is not impossible to break down and re-make in- 
stinctive behavior, as the change in marriage cus- 
toms since the days of the cave man shows us. Never- 
theless, it is much easier to change learned behavior 
patterns, one of which these experts believe war to 

We must still take account of individual aggres- 
siveness, and the fact that man appears to be rela- 
tively easy to lead into mass combat. Even if war- 
making is not instinctive, if it is a learned pattern 
of social behavior, there is evidence that it has 
existed for some fifty centuries, and it would prob- 
ably require at least a few centuries of intelligent 
and fairly concerted effort by those who do not be- 
lieve in its utility to unlearn the habit. 

There is a second important set of biological proc- 
esses which at first sight appear to work inevitably 
toward the production of war. These center about 
the question of overpopulation, that is to say, about 
the relation human numbers bear to habitable land 
areas. This is the next primary problem which we 
must consider. 

Over the world there is a limited range of habit- 
able land; and thus far we have no intimation of 
any practical method of emigration to neighboring 


and perhaps less occupied planets. And there is a 
rapidly expanding human population, which is even 
now becoming uncomfortably dense in the crowded 
nations. It is often said that this is a fundamental 
cause of tension which makes wars inevitable, as 
hard-pressed dense populations seek food in more 
amply-provided areas. 

The desirable biological results of wars so induced 
have been, and still are, supposed to be two: 

1. The dense populations are thinned to the bear- 
able point as a result of the fighting, or 

2. Superior nations, or races, are victors. They 
expand at the expense of the defeated inferior group 
and so occupy more of the limited space which is 
available for men. 

Let us test these theories against the known facts. 
Roughly speaking, there are about fifty-two million 
square miles of land surface on the earth. (95) This 
includes the habitable plains of the temperate re- 
gions; it also includes the relatively uninhabited 
deserts, tropical jungles, and mountains. Approxi- 
mately one-fourth of these fifty-two million square 
miles is desert or semi-desert and can support only 
a sparse population of men. This leaves roughly 
forty million square miles of non-arid land theoreti- 
cally open to human habitation. 

On this land there are living at present, accord- 


ing to a 1935 revision of the estimated world popula- 
tion which was made by Professor Pearl, something 
over two thousand millions of people. This is almost 
exactly forty people per square mile of the whole 
earth's surface, or about fifty people per square mile, 
if the arid and semi-arid land is excluded. We can 
better visualize the meaning of these figures when 
we know that they are almost exactly the average 
population density for the United States; forty per 
square mile for the whole land area, and fifty per 
square mile if on land with fair rainfall. 

A recent estimate of human population of three 
hundred years ago, tentatively advanced by Profes- 
sor Pearl, is that in 1630 there were probably about 
445 million people on the whole earth, or about 
eight per square mile of total land surface. (95) Dr. 
Pearl thinks that this was probably the largest hu- 
man population which the earth had supported up 
to that time. Then came the opening of the Americas 
for settlement, and the beginnings of modern use 
of transport and manufacturing processes, and the 
scattering of information by modern methods. The 
result has been that in the last three centuries the 
population of the world has increased almost five- 
fold, from eight to forty per square mile, largely 
because food and shelter and mechanical energy were 
made available for five times as many people, and 


because the development of modern science made 
the world safer for them. 

In these three hundred years the world popula- 
tion has doubled, on the average, approximately 
every sixty-four years. Today mankind is increasing 
in numbers at such a rate that if the increase should 
continue as it was going in 1935 we could expect 
another doubling of the number of people in the 
world in approximately seventy years, and we should 
have about eighty people per square mile in the 
year 2005. 

What then? Will not the coming generations at 
some time be obliged to fight for their place in the 

This prospect is somewhat altered, however, by 
the fact that many students of population trends 
believe that the rate of human increase is slowing 
down. In the case of the United States, Dr. Baker, 
(20) economist of our Department of Agriculture, 
has estimated recently that unless present trends are 
changed (and they may be) there will be a further 
population increase in the United States of only 
about eight million in the next two decades. He 
thinks that the population will then have reached 
its maximum, if conditions remain as they are today. 
Thus, according to Dr. Baker, we are looking for- 
ward to a maximum population of less than 150 


million people, or less than fifty for every square 
mile of our country. Others put the figure higher, 
but I know of no expert who expects our Ameri- 
can population to double itself again unless there is 
a radical increase in available energy or in other 
aspects of our living conditions. 

For the world as a whole. Dr. Pearl estimated in 
1936 that, if present trends continue, as they may 
not, the world population will reach a maximum of 
about 2,650 millions by the year 2100. (95) This is 
a density of about fifty persons per square mile of 
land surface on the globe, counting good and bad 
land alike. 

I must dissociate myself from any responsibility 
for these and similar estimates. I fully realize, as do 
their authors, the pitfalls inherent in such predic- 
tions. Human trends being what they are and have 
been in the last three hundred years, this is as good 
an approximation as can be made at present, and 
with all its faults it is worth considering. 

The important aspect to me is that we do not 
have reason to expect in the United States or in the 
world a continuation of the unprecedented rate of 
increase of the last three centuries, or even a con- 
tinuation of the present rate of increase. Unless 
population experts are all at fault, the rate of re- 


production among human animals is slowing down, 
just as the rate of increase in non-human popula- 
tions slows down as laboratory containers approach 
an overcrowded condition. In fact, few animal popu- 
lations approach the limits of their food supply in 

The reasons for this are not clear, though they 
appear to be connected with the ease of securing 
available energy, food and shelter. As men approach 
the bearable limits of these necessities of life there 
occurs an increase in birth control. This is shown 
in Italy, where, according to figures given in The 
Statesman's Year Book, (114) despite continued 
propaganda for a higher birth rate the actual num- 
ber of births fell over 12 per cent from 1922 to 
1936 (Figure 43). Thanks to a similar decline in 
death rate the significant percentage of births which 
are canceled by deaths has remained fairly steady. 
In England, where there has been no great effort to 
encourage population increase, the deaths in 1922 
were 62 per cent of the births; in 1935 they were 
81 per cent. Perhaps the success of Italian efforts is 
to be measured by this comparison with England 
rather than by the fact that under propagandist pres- 
sure their birth rate has actually decreased. In Ger- 
many, the present regime has not been in power 
long enough to establish a trend. The graph (Fig- 



ure 43) shows that beginning in 1934 there has been 
a dramatic decrease in the percentage of births can- 
celed by deaths; actually there has been a decided 
increase in births. Recent analyses in the American 

1928 1929 1930 1931 1932 1933. 1934 1935 

Fig. 43. The percentage of births that were canceled 
by deaths for the given years in Italy and Germany. The 
higher the trend line, the slower the population is grow- 
ing and vice versa. The broken line connects the ob- 
served points; the solid line shows the mathematically 
smoothed trend line. (Data from The Statesman's Year 


Journal of Sociology indicate that the present opin- 
ion is that the increase in the birth rate may be 
the result of a campaign against abortion which in 
pre-Nazi times terminated over one-third of the 
pregnancies. (58, 125) One can deduce from general 
biological experience, despite the current German 
data, that the population almost automatically ad- 
justs numbers within its physical and biological 
limitations. Doubtless eventually this mysterious 
process of population adjustment will be analyzed. 
At present we have made some progress toward an 
understanding of the factors involved in non-human 
populations, but have little objective knowledge to 
report where men are concerned. 

It is of course possible to increase the present 
food supply of the world enormously. It has been 
estimated that if our present biological knowledge 
were consistently applied we could raise food enough 
to supply at least ten times the present world popu- 
lation, instead of the 25 per cent increase to which 
we are looking forward by the year 2100. Presum- 
ably by that time we shall have learned much more 
than we now know about intensive methods of food 

Let us take one simple instance only. In the 
United States we are substituting gasoline-driven 
farm machinery for horse power in agricultural work. 


The land required to produce feed for one horse 
will equally well provide food for a man. Baker, 
the agricultural economist cited earlier, estimates 
that the land released annually by this change in 
farm technique can be turned to growing human 
food almost as fast as our population is increasing. 

The question seems rather one of adequate food 
distribution than of shortage of food. Under con- 
ditions which we can visualize at present there seems 
little likelihood of a real food shortage for the world 
as a whole. 

If, however, these conclusions prove to be com- 
pletely wrong, and the world population is now or 
will become too high by biological standards, there 
is still the question as to whether war is a sound 
and sufficient means of controlling population 
growth. The theory that war is an efficient means 
of stopping the increase of mankind is so contrary 
to fact that I allow myself to say No in the first 
place and present the evidence later. 

The immediate effect of a war upon the civilian 
population is to depress the birth rate and raise the 
death rate on both sides of the line, whether in the 
winning or the losing nation. Figure 44, taken from 
a study by Pearl on population trends during the 
World War, gives these data for the unoccupied parts 



of France, for Bavaria and for England, from 1913 
to 1918. (92) 

In 1913 deaths and births in these parts of France 
were almost equal; in 1918 there were approximately 


Fig. 44. The percentage which deaths were of births 
steadily increased during the war years in France (non- 
invaded departments), Prussia, Bavaria, England and 
Wales. (From Pearl.) 

two deaths for each birth. In Bavaria, in 1913, there 
were five births for every three deaths; in 1918 there 
were three births for every four deaths. The trend 
lines in Figure 44 for these two countries run 
almost parallel, though France was invaded and los- 
ing in much of the fighting while Bavaria was free 
from foreign troops and part of a winning nation 
until near the end. As usual, analysis of such a situ- 
ation is not simple. Bavaria, although enjoying the 


psychological advantage of belonging apparently to 
the winning side, suffered the physiological disad- 
vantage of an increasingly severe food shortage, while 
France averaged an adequate food ration. In Eng- 
land during the same time, where there was neither 
invasion nor starvation, there was the same tendency 
toward increase of deaths in proportion to births, 
though less marked. These statistics, of course, do 
not take into account the almost unprecedented 
death rates in the fighting lines. 

Temporarily the population growth was checked, 
but almost immediately following the close of the 
war the ratio of births to deaths resumed their pre- 
war trend lines. Pearl, writing in 1921, (93) summed 
up his study in these words: "Those persons who see 
in war and pestilence any absolute solution of the 
world problem of population . . . are optimists in- 
deed. As a matter of fact, all history tells us, and re- 
cent history fairly shouts in its emphasis, that such 
events make the merest ephemeral flicker in the 
steady onward march of population growth." 

Fifteen years later, in 1936, (94) Pearl again wrote, 
alluding particularly to the effects of wars of con- 
quest by one nation to acquire the territory of an- 
other: "The world problem of population and area, 
however, remains unaltered in theory, though prac- 
tically it will have been made worse because of the 


extravagantly wasteful destruction of real wealth that 
war always causes. This is the problem that is really 
serious— how can forty persons be maintained for 
every square mile of land surface of the globe- 
good, bad and indifferent land together? War can- 
not enlarge the land surface that must support man- 
kind; it has never diminished the total number of 
people who want to live on it except by a tiny frac- 
tion for quite a brief period. There is no way out 
of the dilemma by the pathway of war." 

It is a comparatively new idea that population can 
be controlled at all except by famine, pestilence, 
and war, which have been regarded as acts of God. 
Acts of God or not, we can no longer tolerate famine 
or pestilence if we have the power to prevent them; 
and lacking such power we intend to get it as soon 
as it is humanly possible. Among dispassionate, ex- 
pert students, war has similarly lost caste as a means 
of population control, though the man in the street 
has not yet learned this. 

Instead of the dubious check these agencies fur- 
nished there is a steady turning to birth control, 
even in the countries where it is most surprising to 
find this. In Germany and Italy, although artificial 
stimuli are being applied to keep up the birth rate, 
some kind of birth control evidently is occurring. 

There is significance not only in the average 


density of people per square mile of the earth's sur- 
face, but also in the population density of the most 
crowded nations. The degree of crowding in cer- 
tain countries with whose problems we are familiar 
is shown in the following list. The figures given are 
slightly rounded statements of the average popula- 
tion density per square mile of land territory. The 
most densely populated countries of the world are 
listed here in order (94): 



1. Belgium 


2. England and Wales 


3. Netherlands 


4. Japan 


5. Germany 


6. Italy 


7. China (proper) 


8. Czechoslovakia 


For many purposes it is hardly fair to compare 
the relatively small countries like Belgium and the 
Netherlands with others like Japan or Italy which 
are larger but contain a high percentage of waste 
land. For our purposes, however, the list as it stands 
is fair enough; such data represent the facts we have 
to face. 

At present about two and a half acres are required 


to supply food to one person, if the soil is fair to 
good and the husbandry is good according to present 
standards. This means that under modern condi- 
tions of agriculture the upper limit of a relatively 
self-contained population is about 250 people per 
square mile. It will be seen that Belgium with its 
700 per square mile almost triples this upper limit, 
and that England and Wales and the Netherlands 
more than double it. Such high population densities 
can be supported by trade conducted with other 
countries on a large scale. They could also, as we 
have seen earlier, be supported by improved meth- 
ods of agriculture. An Italian expert on populations 
said in my hearing some years ago that population 
pressure is not a direct cause for war, but can be 
used by a clever leader to range a nation behind 
aggressive policies which lead to war. In the short 
run that is easier than to educate people to apply 
the available knowledge which would allow Italy, 
for example, to feed her present population, and 
more, from the products of her own soil. 

It is time now to turn to the second of the ques- 
tions concerning the biological background of war. 
In the light of the preceding discussion we can re- 
state this question as follows: Although underlying 
biological relationships do not necessarily lead to 


war, is not war biologically justified by the results 

If war does benefit the race in distinct and unique 
ways, then the biologist must favor a system of so- 
ciety which will bring about the proper kind and 
the correct number of wars to produce the best racial 
selection. If war, on the other hand, tends toward 
human deterioration then the biologist must oppose 
a system of international relations based on war. 
Again it is a question of evidence. 

The matter of individual biological selection is 
one that is fairly obvious even to the layman; and 
his conclusion that the direct results of war are harm- 
ful biologically has been well supported by scientists 
whose interest in the subject is more inclusive than 
their natural sympathy for the young men of their 
acquaintance who have incurred wounds or have 
been gassed or have suffered severely from some of 
the typical wartime epidemic diseases. The work of 
David Starr Jordan before 1914 is classic; (70, 71, 
73) but the evidence furnished by the World War is 
more important to us. American experience at that 
time is best set forth in the slender book by Professor 
Harrison Hunt (67) of Michigan State College, who 
studied the records of the American army, using mod- 
ern statistical methods. 

He was left with no doubt that war selects the 


best of our young men for exposure to wartime haz- 
ards. We have space for one bit of evidence. Hunt 
found that for the drafted American army, 83 per 
cent of the mentally defective were rejected; those 
of normal mentality and the 17 per cent who were 
only slightly subnormal were held for service. A 
good geneticist would have reversed the procedure, 
sending the mentally deficient out into wartime risks 
and keeping the others at home to continue the 
race. But this is so contrary to fact in all the stand- 
ards by which armies are selected that it seems faintly 
ridiculous in the telling. Personal selection, so far as 
it exists in modern warfare, selects the individual to 
be killed or wounded because he is physically or 
mentally superior to those who are left at home. (64) 
The ill effects of this selection among the young 
men are evident in a nation where war losses have 
been heavy, but they are less drastic for people as 
a whole than they might be if it were not for various 
mitigating factors. To date only half the race has 
suffered in so-called civilized warfare, since women 
have been exempt from actual combat. Also many 
young men return who, though wounded and per- 
haps otherwise handicapped, are still physically ca- 
pable of passing on their gerra plasm to succeeding 
generations. And even in populations badly shattered 
by war most of these genetic ill effects could be ob- 


viated if monogamy were less of an ingrained human 

The effects of severe wartime epidemics, which 
are usually the cause of more deaths than the actual 
fighting, are subject to the same comments; but with 
these epidemics the civilian population is also di- 
rectly affected, as was the case with the influenza 
pandemic that swept the world in 1918, and carried 
off in a day more civilians than did many spectacu- 
lar air raids combined. 

General epidemics tend to fall most heavily on 
the old and the young; biologically we are most in- 
terested in the fate of children and young people. 
Disease and undernourishment drastically reduced 
the younger population in places well away from the 
fighting lines in the last war. Homer Folks, (47) U. S. 
Red Cross commissioner, testifies that in some sec- 
tions of Italy 60 per cent of the children failed to 
survive wartime conditions. The children of Ger- 
many and of Poland suffered greatly. 

If he could know that such severe exposure elimi- 
nated the relatively weaker specimens and left a 
stronger, hardier race, the biologist could reconcile 
himself to the death of these children, though emo- 
tionally he might rebel. 

But this rationalization is impossible. Study of the 
after-effects of epidemics upon children (45) does 


not show a group of sturdy survivors, with all the 
weaklings eliminated. Rather, the later history of 
these children shows that they have a lower resistance 
to the next severe disease that strikes them. Ap- 
parently many such children, though surviving, are 
weakened for some years thereafter. 

Similarly, the children back of the battle lines in- 
clude many whose experience left a mark, and who 
recover only slowly from its injurious effects. They 
were not a selected lot, and their own generation 
has suffered. Fortunately all our evidence indicates 
that those who survived are able to pass on their 
inherited qualities unimpaired to their children; but 
many are unable to provide for their families the 
physical care and conditions for living which make 
for the fullest development of inherited potentiali- 

Perhaps a sane and cautious quotation from Pro- 
fessor Holmes of California will be a fitting sum- 
mary for this section. In 1921, Holmes wrote: (63) 
**On the whole it is quite probable, I believe, that 
the effect of military selection is harmful. ... It is 
a matter of serious doubt whether the beneficial fac- 
tors come near outweighing the adverse selection of 

What are some of the beneficial effects which this 
statement suggests may exist? One of them is that 


war is necessary to maintain racial vigor. This is a 
matter on which statistics are not available, and on 
which personal opinion must play as reasonable a 
part as it can. 

To me it seems a misreading of history that leads 
to the justification of war as a means of keeping up 
the vigor of the race. I should say, rather, that wars 
have frequently revealed the loss of racial or national 
vigor among a people made soft by easy living, which 
in turn had been made possible, at least at times, by 
a long series of successful wars of conquest. 

Anyone who attempts to maintain the thesis that 
wars do keep racial stocks vigorous— and there are 
biologists who believe this— is troubled by the Chinese 
people. This much-discussed and frequently invaded 
land was populated by the forerunners of the pres- 
ent Chinese during the days when Egypt, Assyria, 
Babylon, Greece and Persia, to name no more, were 
fighting the wars recorded in our general histories. 
Those warlike peoples have lost their racial vigor 
but the Chinese, who have been relatively peaceful, 
have retained it. This stumbling block cannot be 
removed by denying racial vigor to the Chinese; they 
have, in the past, absorbed too many temporary con- 
querors, and have occupied and are occupying by 
peaceful penetration too much of the earth's terri- 
tory, to be dismissed as a racially decadent people. 


There are anthropologists who reckon them biologi- 
cally the most advanced people living today. 

There is another allied but somewhat different 
theory regarding the human benefits conferred by 
war which holds that even though in direct personal 
selection the war system is dysgenic, it does tend to 
select the fittest races and nations for survival. This 
theory is usually applied to European history, w^here 
in the long struggle of advanced European nations 
against backward poorly-equipped natives of Amer- 
ica, Asia, Australia and Africa, victory has eventu- 
ally rested with the Europeans. Whatever the in- 
trinsic human merits of the case, a question on which 
Hindus may disagree with Englishmen, there can be 
no doubt that such conflicts have been won by the 
nation which possesses the more modern social or- 
ganization and the better gadgets with which to 
fight; and the winning nation has not hesitated to 
levy on the weaker one for whatever of its posses- 
sions and services it could utilize for its own 

When, however, one European nation fights an- 
other, as, for example, France and Germany, who 
can maintain that the nation that won at Waterloo 
and in 1918 is superior to the people who won at 
Leipzig and Sedan? Or, to come closer home, does 
the fact that the Confederacy lost the war between 


the States prove that the white people of the South 
are racially inferior to those of the North? 

Actually, of course, we are not fighting racial wars 
at present. What race won the World War, or for 
that matter, lost it? Modern warfare among so-called 
civilized powers probably does result in victory for 
superior wealth, better organization, shrewder propa- 
ganda, and other social achievements, but we have 
little good evidence to link these social attributes 
with racial stock, in spite of contemporary German 
determination to assume the connection. 

Let us allow Popenoe and Johnson, (99) recog- 
nized students of eugenics, to summarize this whole 
inquiry into the biological justification of war. Writ- 
ing in 1918, when the subject was near the top of 
men's minds, they said: "When the quality of the 
combatants is so high compared with the rest of the 
world as during the Great War, no conceivable gains 
can offset the loss. It is probably well within the facts 
to assume that the period of the late war represents 
a decline in inherent human quality greater than 
in any similar length of time in the previous his- 
tory of the world." 

It seems to me that such evidence and reasoning as 
I have presented indicates pretty clearly that the 
present system of international relations is biologi- 
cally unsound. Attempts which have been made in 


the past to lend biological respectability to the pres- 
ent system by regarding it as an expression of an 
inevitable struggle for existence have overlooked not 
only its defects as a selecting agent but, more serious, 
have often not even been conscious of the existence 
of another fundamental biological principle, that of 
co-operation. Is it possible to envisage a system of 
international relations which will be fairly based on 
both these aspects of biology? 

One of the first questions to be examined is that 
of the size of the co-operating unit practicable in 
such a system. It is possible to make a case for the 
present human social divisions, where nations of var- 
ious size co-operate within their own boundaries 
though competing with each other for various types 
of supremacy. Within each of these nations are 
graded series of groupings in great variety, which 
also co-operate within and compete across their tan- 
gible or intangible boundaries. Here immediately 
we come across an important qualitative difference 
in the competition. Within each nation this inter- 
group struggle is normally carried on by approxi- 
mately peaceful and orderly means. By contrast it 
is accepted that the competition across national 
limits, usually peaceful and orderly, may at any time 
break down into the socially backward phenomenon 
called war; and even in periods of peace and social 


progress much of the average nation's energy, wealth 
and forethought is diverted to preparing for the next 

Peaceful intergroup competition within a nation 
has come to rest, in the first place, on habit, prefer- 
ence and a realization that only temporarily is an 
advantage gained by violence; and, in the second 
place, on a government, often set up by mutual con- 
sent of the competing groups, which is strong enough 
to block or stop cruder appeals to force, and which 
is expected by them to do so. 

The suggestion has been urgently repeated since 
the time of Sully, (61) the great minister of Henry 
of Navarre and France, that there should be a simi- 
lar international organization. Theoretically there is 
almost everything to be said for this proposal. Such 
an international organization might be set up much 
as the federal government of our country was 
planned, to supervise the functioning of the differ- 
ent states. This system calls for representative gov- 
ernment, a relatively unbiased court of final judicial 
appeal, and certain potential police power, which in 
our American experience has been used but rarely 
on a national scale. 

The present League of Nations, even in its most 
hopeful days, did not show more than remote pos- 
sibilities of equaling on a world scale what the British 


Empire has done fairly adequately of recent years 
for more than one-fourth of the earth's land area. 
Any future international body which will undertake 
to apply the balanced principles of struggle and co- 
operation on a global basis must, among its other 
qualifications, avoid certain outstanding mistakes of 
the present League. 

It cannot be really co-operative if it is basically a 
league of victor nations formed to administer a puni- 
tive peace treaty, for this is hardly a step in advance 
of the time-honored national alliances for defense 
and offense, which are co-operative only to be de- 
structive. It must not be dominated in any depart- 
ment by the representatives of any one nation, not 
even when that nation is as intelligently, and shall 
I say selfishly, benevolent as England and its domin- 
ions today. It must be so organized as to secure and 
hold adherence from the great majority of nations. 
As a step toward this end, the biologist's international 
system must be a dynamic organization capable of 
and designed to effect changes rather than set up 
to preserve any given status quo, regardless how 
favorable for the predominant powers. 

Biology teaches the inevitability of change, if it 
teaches anything. We must have some device in our 
system which will allow for needed changes, some 
means of making those compromises at which the 


English and the French are so proficient in their in- 
ternal affairs. In international as in legal circles, we 
must have some peaceful means of declaring a de- 
funct nation to be in fact bankrupt or unable to 
manage its own business, and to distribute its assets 
among the proper creditors. 

When such a system is installed there will need 
to be not only the means for international consulta- 
tion, and a hearing for the troubles of the world; 
there will also be the necessity for courts of inter- 
national justice. One of these may well grow out of 
the present World Court at Geneva, patterned on 
the Supreme Court of this country; another might 
be a development of the international court of arbi- 
tration which has been located for many years at 
The Hague. 

At this point we come to a serious divergence of 
opinion. Should these courts be supported by police 
power? As a realistic biologist it seems to me that in- 
ternational police force will probably be a necessity 
in those cases when a nation or a section of a nation 
attempts to raise itself in the peck-order of govern- 
ments by direct action rather than waiting for the 
results of the more just but slower pressure of world 
opinion. Much of the police activities should be 
limited to such duties as are now exercised by our 
federal marshals, but in my judgment there would 


need to be the possibility of the use of even stronger 
police pressure. 

But it is certain that if an international organiza- 
tion is to succeed, police power must be used very 
rarely. The attempts of the British government to 
coerce the American colonies or the Irish people are 
conspicuous as a demonstration of the frequent fail- 
ure of massed force to compose complex human 
maladjustments. It is noteworthy that such enforce- 
ment has not been used in the long and successful 
operation of our own Supreme Court. 

Practically, it is possible that nations will join in 
an international enterprise which is limited to con- 
sultation and judicial review of all disputes long be- 
fore they will relinquish any other phase of their 
jealously guarded sovereignty to such an interna- 
tional organization. We may even be able to work 
out a method of international co-operation based 
entirely on patience, wisdom and justice, though in 
the light of past experience this seems at present 

Such a world organization will never be perfect. 
Man is not. Neither is the government of Chicago, of 
Illinois, of our United States. And yet who would 
not prefer to live in Chicago, even back in the gang- 
ster era of the nineteen-twenties, rather than in the 
period of greater individual freedom for privileged 


people that London or Paris of the Middle Ages 

A thoughtful and sincere biologist may object that 
the world is too large an area for a successful co- 
operative unit; that we need units intermediate in 
size to allow for human evolution those advantages 
which Professor Wright has demonstrated for popu- 
lations intermediate in size. To such objection one 
must reply that, as to the latter point, the main- 
tenance of smaller co-operative and competing units 
within the larger one is part of the scheme as 
sketched. And to the first, that of the great size of 
the earth, it needs only to be mentioned that thanks 
to recent improvements in transportation facilities. 
New York is in point of time as near the Orient as 
it was to Los Angeles in 1885; and there are few 
places on the globe as remote from Washington as 
was San Francisco before the Union Pacific Railway 
was built. In transportation and communication, and 
in community of essential human interests, the world 
is ripe for a workable international organization. 

From the standpoint of pure biology, disregarding 
considerations that may seem to smack of the social 
sciences, the mortal enemies of man are not his fel- 
lows of another continent or race; they are the aspects 
of the physical world which limit or challenge his 
control, the disease germs that attack him and his 


domesticated plants and animals, and the insects that 
carry many of these germs as well as working notable 
direct injury. To the biologist this is not even the 
age of man, however great his superiority in size 
and intelligence; it is literally the age of insects. (7) 

This is a fact which must have repeated emphasis. 
In the tropics there is only the narrow strip along 
the Panama Canal and similar small areas in which 
man has shown the ability to compete successfully 
with the insects; and the techniques of this competi- 
tion are too expensive as yet to apply along the vast 
rich stretches of the Orinoco River, the Amazon or 
Congo; there, undoubtedly, the insects are in con- 
trol. In countries like India and Russia mosquito- 
borne malaria is a plague which saps the energy of 
those enormous populations as it does today in our 
own South. 

There are good biological precedents for such 
competition between different types of organisms as 
that between man and insects or betw^een man and 
bacteria. In fact, with almost negligible exceptions, 
the only kind of mass slaughter for which there is 
precedent in animal biology is found in interspecific 
struggles. One species of animal may destroy another 
and individuals may kill other individuals, but group 
struggles to the death between numbers of the same 


species, such as occur in human warfare, can hardly 
be found among non-human animals. 

These techniques by which we can successfully 
combat our enemies, the insects, and the viruses they 
transport are too expensive for the world today. 
They are too expensive because even the peaceful 
nations are using so much of their resources for buy- 
ing and building armament on an unprecedented 
scale, apparently to make one more experimental test 
of the fact that war is biologically indefensible. 

In our struggles with our physical environment, 
with disease germs and insects, we have ample op- 
portunity for the struggle for existence, and stimu- 
lus enough to apply to the limit the principle of 

Unconsciously or consciously, the innate urge to- 
ward co-operation appears even under circumstances 
where it would seem least likely to be fostered. 

Even in the most seriously war-torn countries, as 
in Spain today, when one is withdrawn from the ac- 
tual scene of battle one finds the common people en- 
gaged as best they can in their normal activities of 
providing food, clothing and shelter for themselves 
and their families, with the ineradicable drive to- 
ward constructive co-operation that we have found 
evident throughout the animal kingdom. Such co- 
operative activity will reach through a family, from 


family to family, from city to city and even across 

These normal activities can be wiped out in a few 
minutes by the exaggerated expression of the struggle 
for existence which we call war, extended beyond 
all biological justification and become, as Malinowski 
has said, "nothing but an unmitigated disease of 
civilization." (78) 

It is a disease of long standing which even under 
most favorable conditions we must not expect to 
see cured overnight; but the outlook is not without 
hope. There seems to be no inherent biological rea- 
son why man cannot learn to extend the principle of 
co-operation as fully through the field of interna- 
tional relations as he has already done in his more 
personal affairs. In addition to the unconscious evo- 
lutionary forces that play on man as well as on other 
animals, he has to some extent the opportunity of 
consciously directing his own social evolution. Un- 
like ants or chickens or fishes, man is not bound 
over to form castes or peck-orders or schools, or to 
wait for a reshuffling of hereditary genes before he 
can discontinue behavior which tends toward the 
destruction of his species. 


Social Transitions 

WHEN DOES an animal group become truly social? 
This question has already arisen in preceding chap- 
ters and is difficult for a thoughtful biologist to an- 
swer with confidence. 

One school, now happily small, regards society as 
beginning when animals first display a social in- 
stinct. (16) By this they probably mean that social 
animals have inherited a behavior pattern that 
causes them to live together with others of their kind 
in more or less closely co-operative units. Others 
consider that animals are social when they carry on 
group life in which there is clear evidence of a divi- 
sion of labor. (42) There is also the frequent sug- 
gestion that only those animals are truly social whose 
behavior is an extension, directly or indirectly, of 
familial behavior. (119) 

For myself, I regard those groups in which ani- 
mals confer distinct survival values upon each other 
as being at least partially social; this is the concep- 
tion that has most often appeared in these pages. (3) 



And from a still different point of view, those who 
would stretch the idea of social living rather widely 
would say, as I have indicated in Chapter V, that 
when animals behave differently in the presence of 
others than they would if alone, they are to that 
extent social. (115) 

These ideas concerning what constitutes a proper 
definition of animal societies, while not necessarily 
mutually exclusive, are sufficiently different to raise 
difficulties when one tries to examine critically the 
useful general concept of social life; it will be profit- 
able to study some of them separately. 

As to the first definition, that social life must be 
limited to those animals that possess a social instinct, 
an inherited behavior pattern, it is hard to demon- 
strate beyond reasonable doubt that many patterns 
of social behavior are in fact inherited. Is the 
tendency of many fishes to form closely-knit schools 
inherited or an early-conditioned bit of behavior? 
There is some evidence that it is inherited, but we 
are not yet sure of it. But if it were granted that 
such schooling tendencies are innate, it would not 
necessarily follow that they are instinctive. There are 
different degrees of complication of inherited be- 
havior patterns, from the relatively simple reflex ac- 
tion of an unborn embryo to the complex mating 
behavior shown, for example, by some insects and 


by rats. The exact determination of the place in this 
line of increasing complexity at which an action 
ceases to be a simple reflex and becomes a more 
elaborate tropism, or the point at which the tropism 
gives way to an instinct, has never been made. That 
is, we do not know just how far down in develop- 
ing patterns instinctive behavior extends. 

There is the added complication that the word 
"instinct" has been loosely used. The most workable 
definition that I have arrived at is a modification of 
an older one of Wheeler's: An instinct is a com- 
plicated reaction which an animal gives when it re- 
acts as a whole and as a representative of a species 
rather than as an individual, which is not improved 
by experience, and which has an end or purpose of 
which the animal cannot be aware. Too frequently 
the word has been applied to any bit of behavior 
whose origin and motivation the observer did not 
understand, with the unfortunate paradoxical im- 
plications that thereby the action was explained and 
at the same time could not be further explained. 
As a result of this uncritical usage many careful 
workers disapprove employing the word under any 
conditions, and particularly in the field of social 

In recent years some students of social life have 
attempted to avoid the term "social instinct," while 


employing the same fundamental idea under the 
thin disguise of "social appetite," (122) "social drive," 
or "group interattraction," (100) which is apparently 
understood as inherited. These contributions to a 
more picturesque language do not necessarily ad- 
vance our understanding of social behavior. 

Still others sincerely believe that fiehavior patterns 
are not inherited, which seems to me a clearly un- 
tenable position. But however strong my belief in 
the actual inheritance of social behavior I do not 
consider it helpful to make the possession of such an 
inheritance the major criterion of social living; it 
is not a practical working test as to what constitutes 
social life. 

If division of labor be used as a touchstone the 
same type of difficulty arises. We do not know how 
to determine when such a division becomes suffi- 
ciently general to merit being called a social attribute 
in the stricter sense in which we are now using the 
term. For example, there is a division of labor which 
is associated with sex and which is almost as exten- 
sive as sex itself. When does this particular division 
of labor cease to be merely an expression of sex and 
become social in the commonly accepted use of the 

The mention of sex brings up again another im- 
portant definition of social life among animals which 


has already been listed. This states that only those 
groups which have grown out of the persistence of 
sexual and more especially partial or completely 
familial relations are truly social. This point of view 
has been touched upon with some sympathy in the 
first chapter. There is an important relationship 
which underlies this definition; many highly organ- 
ized social groups do develop from the continuation 
and extension of family ties. But though this con- 
dition has given rise to many of the better devel- 
oped social units, care must be taken not to regard 
its presence as the essential difference between the 
social and the sub-social. As Professor Child (32) has 
suggested, boys' gangs, girls' cliques, and men's and 
women's clubs present difficulties to one who wishes 
to define all societies as extensions of familial rela- 
tionships. It is quite possible to regard such social 
phenomena as expressions of other aspects of the 
social urge which have developed independently of 
paternal or fraternal interactions. There are counter- 
parts of these human groups among other animals, 
as well as counterparts of the extensions of family 
life. The overnight aggregations of male robins, the 
long-continuing stag parties of male deer outside the 
short rutting season, (38) the flocks of mixed species 
of birds common in tropical regions (Beebe tells of 
one made up of twenty-eight individuals represent- 


ing twenty-three species, (24) ) schools of fishes, and 
the swarms of animals spoken of in the second chap- 
ter, all of these instances test and stretch in varied 
ways the idea that only those continuing aggrega- 
tions of animals which grow out of sexual and 
familial interrelations are truly social. 

Inherited behavior patterns, the forerunners of 
instincts, and sexual differences extend down to the 
protozoa; so do continuing family groups, especially 
in the form of structurally connected colonial or- 
ganisms. Group survival values are present in groups 
of organisms in which sex has not yet evolved, as 
well as among those in which sex is elaborately de- 
veloped. In the light of such considerations it be- 
comes exceedingly difficult to establish any one line 
above which life is to be regarded as truly social 
and below which we have only differing degrees of 
sub-social relations. Here, as happens so frequently 
in biology, we are confronted with a gradual devel- 
opment of real differences without being able to 
put a finger with surety on any one clearly defined 
break in the continuity. The slow accumulation of 
more and more social tendencies leads finally by 
small steps to something that is apparently different. 
If we disregard the intermediate stages the differ- 
ence may appear pronounced, but if we focus on 
these intermediates it will be only for the sake of 


convenience that we interrupt the connecting chain 
of events at some comparatively conspicuous link 
and arbitrarily make this the dividing point, when 
one is needed, between the more and the less social. 
It must be recognized that any such division is a 
matter of convenience rather than a natural break in 
the development from mass or simple group behavior 
to highly evolved social life. 

For our purpose in the present account it is suffi- 
cient to recognize that the well-integrated social 
systems of man and other mammals, of bird flocks 
and of insect colonies, exhibit among them the 
highest expressions of social abilities that have 
evolved. In the range of social development shown 
in these animals we find attributes that are truly 
social in the most exclusive use of the word. But 
these highest expressions of social living have their 
roots in tendencies that in the form of unconscious 
co-operation accompany animal aggregations extend- 
ing throughout the whole animal world, as well as 
to some extent among plants. Conceding then the 
difficulties in the way of making any exact definition 
of social behavior, I wish to present some of the 
social implications of mass physiology, particularly 
among well-integrated societies of animals. 

One of the characteristics of social life among the 
insects is the presence of castes (121) which perform 


different functions within the colony. With many 
social insects the division of labor has developed 
to such an extent that the animals which do dif- 
ferent work have bodies that are more or less struc- 
turally appropriate to their principal tasks. The 
reproductive female has a greatly enlarged ab- 
domen; the soldier grows up to possess large jaws 
and heavy armor or other protective and attacking 
devices; a worker may be large or small or medium 
in size, according as its size will best suit for some 
of the varied tasks necessary for the life of the whole 
colony. The situation is greatly different from that 
among human social castes, where a member of the 
aristocracy may be as husky of body and as empty 
of mind as the most menial of the working caste. 

The only physically distinct castes to be found in 
man and the higher vertebrates are those associated 
with sex. In sexual forms there is always a division 
of labor with regard to the primary sexual functions 
except in those rare cases, usually low in the evo- 
lutionary scale, which at one and the same time are 
both male and female. With many, aside from pro- 
ducing eggs rather than sperm, it is difficult to find 
a division of labor or of appearance between the 
sexes. With others, particularly among the more 
specialized animals, there are differences in sexual 
behavior and responsibilities which are associated 


with the more fundamental distinctions of sex. Fre- 
quently, as in man, these differences have developed 
into fairly distinct behavior patterns for the two 
sexes, until each sex is practically a distinct caste, 
almost in the sense used in discussing castes among 
the social insects. 

Sex is usually determined by differences in he- 
redity which are associated with the combination of 
chromosomes (37) and of the bearers of heredity 
(genes) that are found in the sperm and egg whose 
union gives rise to the new individual. Such deter- 
minations occur at the time of fertilization and sex 
is normally unaltered thereafter. 

Exceptions occur which demonstrate that for cer- 
tain animals this normal means of sex determination 
can be overruled by environmental differences. Many 
of these cases are interesting and significant but their 
full consideration here would draw us off the main 
thread of our present discussion. We shall follow 
only those instances in which changes in sex are 
associated with the near-by presence of other indi- 
viduals, considering here two widely differing cases 
which have been carefully investigated in recent 

Professor Coe (35) of Yale has spent much of his 
time studying the sex ratios and sexual changes in 
oysters, clams, marine snails and other related forms. 


In many of these mollusks he has found that the sex 
ratios vary greatly in different environments, and 
has reached the conclusion that frequently among 
these animals the expression of an innate sexual 
tendency may be in part suppressed or stimulated, 
as the case may be, by the environment in which 
any given animal is living. 

A pertinent case is that of a set of marine snails 
of the genus Crepidula. Three of these "boat-shell" 
snails are common animals in the coastal waters of 
southern New England. Their sexual history follows 
similar outlines. After a juvenile period which is 
essentially asexual, the growing Crepidula becomes 
first a male and then later, sometimes only at long 
last, it transforms into a female. A typical species to 
follow through this transformation is Crepidula for- 

When young, these animals move about, but as 
they become older and larger they settle down in 
one place on a wharf piling or a rock or another 
shell. If the larger, older animals are broken loose 
the soft parts are usually destroyed by some predator 
before they can reattach themselves, leaving behind 
the relatively heavy shell. Frequently they form 
large chains of individuals, of which a simple exam- 
ple is shown in Figure 45. The large, bottom snail 


is dead. Attached to its shell is a large female which 
in summer actively produces eggs. Above her are 
two individuals that are undergoing transformation 
from male to female. Scattered about over these are 

Fig. 45. Crepidula fornicata. (A) A basal female is 
attached to a dead shell (D); two individuals are in tran- 
sition stages and there is one male at the apex; three 
motile supplementary males are in mating position on 
the lower transition individual. (B) same group from the 
left side. (From Coe.) 

four smaller snails which are still functional males 
and which can and do move about. Each male has 
a long slender penis by means of which he transfers 
sperm from his body to an appropriate receptacle 
in the body of the female. Several males may par- 
ticipate in the insemination of a single female. 

The growth of these snails is fairly rapid. A young 
snail hatched out early in the summer may, before 
autumn, become a functional male about 16 mm. 
long, which is about two-fifths the size of a fully 


adult female; during the following year he will 
probably transform into a female. 

The relationships which Dr. Coe observed at 
Woods Hole may be summarized briefly. Some two 
hundred young males were taken from their normal 
surroundings and placed in separate containers in 
the laboratory. Two months later only ii per cent 
were still functional males; 15 per cent had trans- 
formed completely into functional females and the 
other 74 per cent were on their way in that direc- 
tion. Random collections of snails of similar sizes 
which had been left alone in their natural associa- 
tions showed that 85 per cent were still functional 
males and only 3 per cent had fully changed into 

Coe summarizes his work with this and the other 
Crepidulas as follows: "There is no doubt but that 
in each of these three species of Crepidula stable 
environmental conditions tend to prolong the male 
phase of these individuals that are suitably mated 
and sedentary." These points are further illustrated 
in his diagram, a part of which is reproduced in 
Figure 46. 

There is evidence from the earlier work of other 
observers, (54, 87) which these recent studies do not 
entirely replace, that association with a female is 
important for the full realization of the male con- 



dition as well as for its prolongation. With these 
snails the tendency to become first a male and later 
a female is probably determined by heredity, al- 
though the hereditary mechanism which promotes 
such a shift is at present unknown. The point of 
interest for this discussion is that the association 


Fig. 46. As Crepidula fornicata gets older and larger 
it passes successively from the sexually immature 
through the male on into a final female stage. Mated 
males retain that stage longer than if actively motile. 
(From Coe.) 

with others, especially among mated males, tends to 
postpone transformation to the opposite sex. 

Some cases are known in which the presence of 
other animals of the same species determines the 
sex. One of the most thoroughly studied is that of 
the worm Bonellia, (21) in which the sexually un- 
differentiated larva does not, in nature, become the 
small parasitic male unless it is associated with the 
large female. 

Among certain nematode worms which are para- 
sitic in insects, if few eggs are introduced into, for 
example, grasshoppers, (3) most of the resulting 


nematode parasites are females; but if many eggs 
are fed, the nematodes that hatch are almost all 
males. The results are not to be ascribed to a differ- 
ential death rate, for approximately 75 per cent of 
the eggs develop in both cases. 

In Crepidula and Bonellia and nematodes, both 
males and females are always present in a popula- 
tion, though in differing ratios. In cladocerans, how- 
ever, of which Daphnia is an example, the species 
may be carried along for many generations by the 
females alone. They produce eggs which do not re- 
quire fertilization, but which develop directly into 
females that again produce other females like them- 
selves. In these cladocerans the race is usually made 
up of females alone, but at times there is an out- 
break of sexuality; males and sexual females appear 
and the fertilized eggs which result from their union 
are more resistant to adverse conditions than those 
which are ordinarily produced and which require 
no fertilization. These resistant eggs enable the spe- 
cies to survive times of environmental stress, such as 
winter's ice or the drying-up of the ponds in which 
these small crustaceans live. 

In one species of Moina, (5) which has been 
much studied by the biologists at Brown University, 
crowding of the females is an effective method of 
bringing on the outbreak of males and sexual fe- 


males, so that overcrowding may be rated as a time 
of environmental stress. Either by the shortage of 
food, by the accumulation of waste products, or 
from some other cause, the association of many fe- 
male cladocerans together results in the production 
of eggs which have a different prospective potency 
from those the same females, uncrowded, would 
produce; and sexual males and females are the result. 

It is evident from these varying examples that 
even the fundamental matter of sex, with the caste* 
like divisions of labor that result from two sexes, 
may be determined by the close association of ani- 
mals of the same species. There is some reason, 
though perhaps it is slight, for suggesting as in 
Chapter III that sex itself may have grown origi- 
nally out of mutual acceleration in division rates 
when two or more primitive organisms were in close 
contact in small space. The whole matter of sex may 
hark back to some of the basic aspects of mass physi- 
ology which were set forth earlier in this book. 

Sex in its different aspects plays a highly impor- 
tant role in the social affairs of animals. It is inter- 
esting to find that this fundamental cleavage through 
so much of animal life can at times be controlled 
by group relationships. Such considerations serve 
again to emphasize the difficulty of drawing a hard 


and fast line, or even a fairly distinct band between 
social and sub-social living. 

One phase of the social implications of sex has 
escaped general comment. I heard it first mentioned 
by Professor Wheeler. (123) Apparently when there 
is a social difference between the sexes it is the fe- 
males that are the more and the males the less social; 
and the few striking exceptions only confirm the 

Among the social ants, bees and wasps the normal 
affairs of the colony are carried on by the females. 
They produce males only when they are needed to 
fecundate the young virgin females at the time of 
their nuptial flight. The males contribute nothing 
to the protection, feeding or housing of the colony; 
after their one sexual activity they die or are killed 
off, and the females which are lucky enough to 
secure a good nesting site carry on with their female 
offspring until sexual reproduction again becomes 
the order of the day (Figure 47). 

With many of the herds of mammals, the main 
duties of communal life are borne by the females. 
They protect and rear the young and herd together 
to protect each other. The males keep to themselves 
except during the relatively brief period of the 
sexual rut. Even when they join the main herds, as 
in the case of the Scottish red deer, frequently the 


males do not fuse with the others. When danger ap- 
pears during the rut, the stags make off and rejoin 
the females when it is past. After a male is sexually 
spent, frequently before the close of the breeding 
season, he withdraws, and the spent males form stag 

Fig. 47. Castes of the common honey-bee; a, queen; b, 
male (drone); c, worker. (After Phillips.) 

parties which are distinctly less social than the bands 
of females. 

In commenting on the relative sociability of the 
sexes among red deer, Darling says: (38) "Matriarchy 
makes for gregariousness and family cohesion. The 
patriarchal group (among deer) can never be large, 
for however attentively the male may care for his 
group he is never selfless. Sexual jealousy is always 
ready to impinge on social relations leading to gre- 
gariousness. ... I contend that the matriarchal sys- 
tem in animal life, being selfless, is a move toward 
the development of an ethical system." 


The flocks of male birds whose social organization 
we have studied in Chapter VI are more combative 
than the females. The human male writes the great 
poems, builds the great bridges, performs the out- 
standing scientific research; but he is also the crim- 
inal, the war-maker, the disturber of the peace. It 
is the human female that is the highly social force 
with our species, and in this we are again similar 
to the others mentioned. 

Among the social animals only the termites have 
fully socialized males; with them the male reproduc- 
tives consort with the female throughout life. Half 
the soldiers are males and the other half are females, 
and so are the workers. Termites are lowly insects, 
but in this one trait they lead the world. No one 
knows how the socialization of male termites was 
brought about, and if we should learn their secret 
it probably could not be applied directly to human 

When we turn from the far-reaching division of 
most animals into two sexual castes to explore the 
origin of the more specialized castes of insects, we 
find two different essential kinds, the reproductives 
and the sterile types. With bees, ants and wasps, for 
example, the usual reproductive females can pro- 
duce eggs without being fertilized by a sperma- 
tozoan. Such eggs always give rise to males. From 


the Store of sperm which she received in the nuptial 
flight the same female can allow her eggs to be fer- 
tilized; such fertilized eggs become females. 

We have seen the comparative unimportance of 
the males. Although the active colony is usually 
composed of females only, these may be quite dif- 
ferent in appearance and function. Typically there 
are the reproductive females and the sterile ones. 
Among the ants the sterile females are divided into 
the protective soldiers, whose main function is to 
protect the colony from the attack of other species 
of animals, and the workers proper. The ant work- 
ers are subdivided on the basis of size (Figure 48). 

Professor Wheeler made the study of these social 
insects, particularly the ants, his life work. In a 
small book, published in 1937 after his death, he 
reaffirmed his belief that ants and bees have evolved 
from ancestral wasps, and that each has developed 
the caste system independently. (124) 

With bees and wasps, whether a given fertilized 
egg is to produce a worker or a sexual "queen," bet- 
ter called a reproductive female, depends on the 
treatment and food which is given to the grub 
which hatches from the egg. If she receives plenty 
of food and is given space in which to grow she 
becomes fully matured sexually; if fed less and kept 
more crowded she becomes an incomplete female 

Fig. 48. Some ant castes: a, soldier; b, form interme- 
diate between soldier and worker; c, worker; d, form in- 
termediate between soldier and worker; e, queen that 
has shed her wings; i, winged mal^. (After Wheeler.) 


and is known as a worker. Apparently the funda- 
mental difference can be brought about only by the 
treatment which the developing grub receives after 
hatching, and is not a matter of heredity. Just how 
the workers are stimulated to give one or more grubs 
the treatment that will allow them to develop their 
full reproductive capacities is not fully known. If, 
however, the queen bee dies or is removed from the 
colony, workers will start enlarging one or more of 
the cells which contain developing grubs, change 
their care and feeding and so allow them to trans- 
form into fertile reproductives. Perhaps they are 
kept from doing so when a queen is present by some- 
thing like a social hormone, which there is good rea- 
son for thinking is produced by the even more social 

The mechanism which results in caste formation 
among ants need not be the same as that in wasps 
and bees, since it is generally conceded that they 
had a separate social evolution. For years two theo- 
ries have been promoted as to how ant castes came 
into being. One group of students thought that ant 
castes were determined as were those of bees and 
wasps, by care and food; another group w^as equally 
sure that the differences were hereditary. After con- 
fessedly wavering between the two views in his long 
study of ants. Professor Wheeler in his posthumous 


book presents the evidence which finally caused him 
to decide that with ants the whole matter of caste 
formation is primarily controlled by heredity. 

This is a question which will undoubtedly occupy 
students of ants for years to come. The evidence is 
not all in, and the fact that at present it tends to 
indicate that ant castes are determined by heredity 
makes all the more interesting the instances in three 
separate kinds of social insects of the apparent evo- 
lution of group control of castes after the hatching 
of the egg. To this hasty sketch of the operation of 
group determination of caste in wasps and bees may 
be added that of termites. 

The bees and their allies belong to one of the 
most specialized of insect orders, so that they are 
assigned a high position in the evolutionary tree of 
that class of animals. The termites, miscalled white 
ants, belong to a relatively unspecialized insect order 
related to the cockroaches, and stand low in the 
evolutionary scale among the insects. They have, 
however, reached a high state of social development. 

Unlike bees, ants and wasps, the colony, as we 
have said, is at all times composed of males and fe- 
males in approximately equal numbers. There are 
male and female reproductives. of which three dif- 
ferent kinds are known; these are the so-called first 
form which have wings for a time and engage in a 


nuptial flight, second form reproductives with wing 
buds, and third form, which are wingless; and there 
are the sterile workers and soldiers in which both 
sexes are also represented equally. The colony is 
usually composed of reproductives of some one sort, 
and the two sterile castes (Plate V). 

The controversy as to whether caste formation is 
a result of heredity or of the social environment has 
been as intense with students of termites as among 
students of ants. The trend of present information 
tends to support the theory of control by the environ- 
ment. (27, 75) A certain California termite called 
Zootermopsis has reproductives and soldiers in its 
colonies, but no workers in the accepted sense of the 
term. Their place is taken by the younger nymphs, 
all of which have the possibility of developing into 
one of the reproductive grades or into soldiers. 
When Dr. Castle of the University of California (27) 
set up experimental colonies of nymphs alone, he ob- 
tained in due time one or more pairs of reproduc- 
tives. If the small experimental colony lacked a fer- 
tile male, one of the nymphs developed into that; if 
a fertile female was lacking and a male was placed 
in the colony, a nymph developed into a fertile fe- 
male. If the nymphs in a colony that lacked both 
males and females were fed on filter-paper which 
contained an extract of fertile females made with 

PLATE V. Winged reprodiiciixe caste, soldiers and 
workers of a termite from British Gtiiana. This is one 
of the largest species of termites and is shown life-size. 
A, winged reprodiictives; B, soldiers; C, workers. (Pho- 
tograph by AVilliam Beebe.) 


alcohol or ether, the males appeared at the usual 
time, but the females were delayed by twelve or six- 
teen days on the average. 

Ordinarily in Zootermopsis only one soldier ap- 
pears in the first year of the life of the colony. By 
removing the soldier as soon as it appeared in the 
experimental colony it was possible to get as many 
as six soldiers within the time that would ordinarily 
have yielded only one. 

In explanation of these and other similar data 
Dr. Castle expresses his opinion that at the time of 
hatching all nymphs possess three sets of possibili- 
ties to the same degree; namely, they may become 
sexually mature though wingless, they may become 
winged and sexually mature, or they may become 
soldiers. At some stage these chances are narrowed 
to two possibilities: the nymph may become sexu- 
ally mature or it may develop into a sterile soldier. 
Since the reproductive possibility is present in all 
nymphs and since its expression is inhibited by a 
substance produced by a functional reproductive and 
eaten by the nymphs, the absence of functional re- 
productives would allow this potential power to ex- 
press itself. Just what determines that one of the 
first small lot of eggs will become a soldier is not 
known, but it can easily be seen that when one 
soldier has started to develop it too may give off an 


inhibiting influence which prevents other nymphs 
from becoming soldiers. In the normal course of 
events a second soldier appears only when the colony 
has become sufficiently numerous so that the soldier- 
inhibiting substance is spread among so many that 
the effect on any one nymph is weakened; and some- 
thing of the same effect of numbers may explain 
why, in a large colony, many nymphs develop at 
times into sexually mature and winged forms. 

There seems to be a relation to the more gen- 
eralized situation noted earlier. When many animals 
are exposed together to a given amount of alcohol 
or some other toxic material, no one of the many 
may receive any overdose, as will certainly happen 
when one or a few individuals meet the full effect 
of the poison. This type of relatively simple mass 
effect, first discovered in experiments on group phys- 
iology among animals that at the most are only 
partially socialized, apparently turns out to be an 
important mechanism in regulating caste formation 
among these highly social termites; and some simi- 
lar mechanism may control the activity of worker 
bees in producing new queens. It is true that the 
control of caste production is probably not the 
simplest form of physiological mass action, for the 
insects may from time to time become less sensitive 
to such inhibition. At these times, many of the 


nymphs may develop into the winged reproductives 
that swarm forth in the nuptial flight. 

As many know, most termites eat wood which, 
paradoxically enough, they are unable to digest 
although they do obtain their nourishment from it. 
The answer to this riddle is that the termites harbor 
in their alimentary canals several species of flagel- 
late protozoans which can and do change the wood 
into substances which both termites and these flagel- 
lates find highly nutritious. 

From many structural relationships we know that 
termites are close relatives of cockroaches, and studies 
by Dr. Cleveland of Harvard (34) have shown how 
the termite societies may have arisen from the much 
less social cockroaches. Here we have an example of 
one of the many possible connections between highly 
developed social life and the less social state illus- 
trated by the mass physiology characteristic of animal 

Cryptocercus is a wood-eating cockroach which is 
found in decaying wood of the forests of the Ap- 
palachian mountains from Pennsylvania to Georgia, 
and along the coastal mountains in the northwestern 
part of the United States. Like their relatives, the 
termites, these cockroaches feed on wood, and also 
like the termites they harbor wood-digesting pro- 
tozoans in their alimentary tract. These wood roaches 


and many termites cannot live long if deprived of 
their associated protozoa, as can be done by appropri- 
ate treatment in the laboratory. 

The young of both cockroaches and termites hatch 
out without these essential protozoans. The termites 
obtain theirs by swallowing a drop of liquid which 
has just emerged from the anal opening of another 
termite; the cockroaches get their protozoans by eat- 
ing the pellets passed from the alimentary tract of 
molting individuals. Once a cockroach obtains a 
good supply it renews itself. One such cockroach 
or a pair can emigrate to a new log and live there 
for a lifetime. Since, however, adult cockroaches do 
not molt, the young of such an isolated pair, when 
hatched, could not receive the so-necessary intestinal 
protozoa, and hence a pair, if isolated, could not 
found a new colony. Actually the eggs hatch at just 
about the time of the annual molting season when 
the young growing roaches cast their outer covering 
and a part of the lining of their alimentary tract. 
At this time the newly hatched young can obtain 
protozoa readily and thereafter they retain them. 
The habit of living together is necessary in order 
that the growing, molting young may transmit their 
protozoa to the newly hatched nymphs. 

The social situation is still more necessary for the 
termites. With them all the intestinal protozoans are 


lost with each molt, and each time that happens 
each newly molted individual must obtain some of 
the protozoans from another member of the colony 
or it will starve. The newly hatched termites often 
obtain protozoa before they are twenty-four hours 
old, and an artificially defaunated termite, if allowed 
to associate with his normal fellows, is reinfected 
within a few days. With the termites, colony life is 
an absolute essential and only the winged males and 
females, the first form reproductives already infected 
with protozoans before taking the nuptial flight, can 
even start a colony without the presence of others to 
carry the needed cultures of protozoans. 

Many cockroaches which neither eat wood nor 
harbor wood-digesting protozoans reproduce so 
rapidly that given good hiding places and plenty of 
food they aggregate in large numbers, as many 
housewives know. These cockroach aggregations, 
which appear to be formed as a result of tropistic 
reactions to the environment, accompanied by tol- 
eration for the presence of others, permitted the 
wood-roach Cryptocercus to develop the habit of 
passing protozoa from one individual to another, and 
so began the long evolution which has resulted in the 
highly adapted, wood-eating roaches found today. 

The same basic adaptation allowed their relatives, 
the termites, to start on the much longer road they 


have traveled to reach their present state of highly 
developed social life. 

We cannot outline the steps taken very closely, but 
it would seem that in this cockroach-termite stock 
aggregations allowed aspects of mass physiology to 
develop which in turn permitted a closely knit and 
varied social evolution. This is about as near as we 
have yet been able to come to charting a direct and 
obvious truly social development from a slightly so- 
cial or sub-social animal aggregation. 

Among grasshoppers crowding can produce obvi- 
ous structural changes (Figure 49). Certain species of 
grasshoppers found in semi-arid regions, such as 
those of South Africa, have two phases (5) that are 
quite distinct from each other. The phases are suffi- 
ciently different so that in the past they have been 
described as being different species. There is at 
present much evidence which indicates that the 
phase solitaria can be turned into phase gregaria by 
crowding the young nymphs into dense masses. The 
opposite transformation may take place when the 
nymphs of phase gregaria are reared under un- 
crowded conditions. The differences between the two 
extend into color, form and size. 

Similarly plant-lice, which are also called aphids, 
exist in wdnged and wingless forms which tend to 
alternate. When the wingless aphids have approxi- 


Fig. 49. The five upper nymphs (1-5) and the lowest 
adult belong to the swarm phase; the others (6-11) show 
different aspects of the solitary phase of the brown 
locust (Locustana pardalina) of SoTith Africa. This is a 
black-and-white copy of a color plate by Faure. Black 
here represents black or bluish-black in the grasshop- 
pers; heavy stippling represents dark brown; light stip- 
pling represents light or golden-brown except in parts 
of Nos. 7 and 9 which are green. 


mately exhausted the juices from one food plant the 
next generation appears with wings; in flying about, 
some of them will usually find a new and suitable 
food plant where they can settle and carry on. With 
some species one of the most effective ways of keep- 
ing wings from developing is to isolate the individ- 
ual aphids and, conversely, one of the best recipes 
for obtaining winged forms is to allow them to be- 
come crowded. (104) 

These distinctly different types of grasshoppers 
and aphids roughly suggest the structural differences 
between the castes of social insects, just as compari- 
son was suggested between the structural differences 
of caste and of sex. The resemblance is so close that 
the line cannot be drawn between its manifestations 
in social and infrasocial animals. Not only that, but 
the mechanisms by which the castes are produced 
appear in many instances to be like those which may 
occur when animals are aggregated together, even 
though the aggregations are below the level usually 
regarded as marking the lower limit of truly social 

And since no one has yet demonstrated the exist- 
ence of truly asocial animals it is impossible to define 
the lower limits of sub-social living. All that can be 
found is a gradual development of social attributes, 
suggesting, as has been emphasized throughout this 


book, a substratum of social tendencies that extends 
throughout the entire animal kingdom. From this 
substratum social life rises by the operation of dif- 
ferent mechanisms and with various forms of expres- 
sion until it reaches its present climax in vertebrates 
and insects. Always it is based on phases of mass 
physiology and social biology which taken alone seem 
to be social by implication only. 

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Aggregations, in nature, 32; 
hibernating, 32; breeding, 
33; migrating, 33, 35, 37; 
various examples, 34; co- 
lonial animals, 41; forced, 
42; feeding, 44; overnight, 
46, 248; relation to social 
life, 49, 272 

Alcohol, mass protection from, 

Alverdes, F., 29 
Ancestral tree of animals, 86, 


Antelopes, 37 

Ants, 26, 32; effect of num- 
bers on digging, 139; im- 
portance of females, 259; 
castes, 262, 265 

Aphids, 272 

Appetite, social, 44, 47, 247 

Arhacia eggs, 69; spermatozoa, 
69, 83; effect of numbers on 
rate of cleavage, 71; effect 
of extracts, 74 

Bacteria, mass protection, 67; 

food for protozoa, 77 
Baker, O. E., 217 
Bats, 36 
Bavaria, population trend, 

Beebe, William, 248 

Bees, 26, 260, 262, 264, 265; 
solitary, 46; importance, 
259; castes, 260 

Beetles, hibernation, 32 

Behavior, of isopods, 20; 
group, §2, 133; social cri- 
teria of, 173 

Belgium, population, 226, 227 

Bennet, Mary, 185 

Birds, 33, 46, 47, 48, 88, 110, 
134' 155' 175' 206, 250, 261 

Birthrate, 219 

Bison, 38 

Bonellia, 256 

Bowen, Edith, 92 

Breeding season, 33, 133 

Butterflies, overnight aggrega- 
tions, 46 

Calcium, protective value, 65 
Canaries, social order, 191, 

193, 207 
Caribou, 37 
Caste, 250, 274 
Castle, G. B., 266, 267 
Chapman, Frank M., 134 
Chapman, R. N., 104 
Chen, S. C, 139 
Chickens, group stimulation, 

135; social order, 176, 190, 

193, 207; IQ, 192 
Child, C. M., 58, 248 



Children, effect of class size, 
143; in wartime, 230 

China, population, 226; racial 
vigor, 232 

Cladocera, sex determination, 

Class size, effect on rate of 

learning, 142 

Cleveland, L. R., 269 

Cockroaches, effect of num- 
bers on rate of learning, 
149; related to termites, 
265, 269 

Coe, W. R., 252 

Collias, N., 185 

Colloidal silver, mass protec- 
tion from, 53, 56 

Colonial organisms, 41 

Community, ecological, 38 

Confusion effect, 139 

Conditioned water, 64, 92 

Co-operation, history, 23, 31; 
ecological, 40; voluntary, 
42; evidence for, 49, 50; un- 
conscious, 88, 133; con- 
scious, 209; principle of, 
209, 211, 242, 243 

Copepods, 35 

Crepidula, 253 

Crowding, harmful effects, 31, 

Ctenophores, 35 
Czechoslovakia, population, 

Cyprinodon, learning, 164 

Daphnia, mass protection, 57, 
139; food for fish, 137; sex 
determination, 257 

Darling, E. Fraser, 111, 260 

Deegener, P., 28 
Deer, 48, 248, 259, 260 
Despotism, 185, 208 
Dionne quintuplets, 201, 207 
Disease in wartime, 224, 225, 

228, 230, 231 
Division of labor, 32, 247, 251 
Dominance, qualities causing, 

190; relation to breeding 

cycle, 194 
Drosophila, effect of numbers 

on rate of reproduction, 


Eggs, sea-urchin, 69 

Elephants, minimal popula- 
tion, 108 

Ellis, Havelock, 24 

Emigration, 122, 126, 131 

Empedocles, 23 

England, population trend, 
219, 223, 226, 227 

Espinas, A. V., 25, 28 

Euglena, 34 

Evans, Gertrude, 73, 92 

Evolution, course of, 86, 87; 
effect of numbers on rate 
of, 118; Lamar ckian, 118 

Family, as origin of society, 
47, 244, 248 

Finkel, Asher, 92 

Fish, schools, 48; mass pro- 
tection, 53, 56, 68; effect of 
crowding on growth, 92; on 
amount of food taken, 136; 
on learning, 158; leader- 
ship, 166; imitation, 170 

Fischel, W., 196 



Flocks of birds, breeding, 111; 
social organization, 175, 
206; leadership in, 196; 
wheeling flight, 198; of 
mixed species, 197, 248 

Folks, Homer, 230 

Forced movements, 43. (See 

France, population trend, 223 

Fresh water, mass protection 
from, 63 

Fundulus, learning, 164 

Gates, Mary, 149 

Gene frequency, 118 

Germany, population trend, 
220, 223, 225, 226; children 
in wartime, 230 

Goldfish, mass protection, 53, 
56, 68; effect of numbers on 
growth rate, 92; on amount 
of food taken, 136; on 
learning, 159; leadership, 
166; imitation, 170 

Gross, A. O., 113 

Group behavior, 22, 133; 
stimulation of feeding, 136; 
organization, 175 

Growth, retarded by over- 
crowding, 91; of goldfish, 
effect of numbers, 92; ef- 
fect of extracts on, 72, 96; 
of mice, effect of numbers, 

Gulls, minimal population, 
110; effect of numbers on 
survival, 111 

Hawaii, snails, 123 
Heath hen, 113, 122 

Henry IV of France, 236 
Hibernation, 32, 58 
Holmes, S. J., 231 
Hormones, effect on social 
rank, 193; social, 264, 267 
Hoskins, Walter, 92 
Hunt, Harrison, 228 

Imitation, 170 

Insects, social, 26, 29, 32, 259; 

population in nature, 39; 

evolution of, 88; as enemies 

of man, 241; castes, 250 
Instinct, 249; definition, 246; 

social, 244, 245, 249 
International relations, 210 
Isopods, behavior, 20 
Italy, population trend, 219, 

225, 226, 227; children in 

wartime, 230 

Japan, population, 226 
Johnson, W. H., 77 
Jordan, David Starr, 228 

Kellogg, John, 185 
Kessler, K. F., 26 
Kropotkin, Prince, 27 

Leader, of a group, 166, 175, 
196, 207; relation to peck- 
order, 199 

League of Nations, 236 

Learning, effect of numbers, 

Liven good, Wayne, 92 

Lobster-krills, 34 

Locusts, migratory, 35; phases, 



Malinowski, B,, 213, 243 
Man, 26; mass protection, 52, 
85, 209; evolution of, 88; 
effects of numbers on men- 
tal work, 142; social rank- 
ing, 201; war, 210; enemies 
of, 240; castes, 251; com- 
bativeness, 261 
Manakin, breeding behavior, 

Mass protection, 52, 85 
Mast, S. O., 81 
Masure, R., 185, 192 
May-flies, 35 
Maze learning, 150; relation 

to social rank, 192 
Metaphysics, 18 
Mice, effect of numbers on 

rate of growth, 99; on rate 

of reproduction, 103 
Migration, 33 
Minnesota, experiments on 

class size, 145 
Mixed flocks, leadership in, 

48, 197 
Moina, sex determination, 

Murchison, C, 182 

Mutation, 118 

Newman, H. H., 206 
Netherlands, population, 226, 

Nichols, J. T., 197 

Paramecium, 76 
Park, Thomas, 105 
Parrakeet, effect of numbers 

present on learning, 155; 

social order, 185, 191, 192 
Patten, W., 27 
Pearl, Raymond, 107, 216, 

218, 222, 223, 224 
Peck-order, 176; relation to 

leadership, 199 
Phases of grasshoppers, 272 
Phillips, John, 107 
Philosophy, 17, 23 
Pigeons, social order, 186, 207 
Planaria, mass protection, 59 
Poisons, mass protection from, 

53' 56 

Poland, children in wartime, 

Popenoe and Johnson, 234 

Population, optimal size, 92, 
103, 104, 125, 128, 130, 131; 
minimal, 108; human, re- 
lation to war, 214; of the 
world, 215, 218; of U. S. A., 
217; of various countries, 

Procerodes, mass protection, 


Protozoa, effect of numbers 
on rate of division, 76; ex- 
planation of effect, 80; as- 
sociated with termites, 270 

Pseudo-leadership, 197 

Oesting, R. B., 92 
Overcrowding, 31, 50; effect 

on growth, 91, 103 
Oxytricha, 77 

Quintuplets, Dionne, 201, 207 

Retzlaff, Elmer, 102 
Robertson, T. B., 75 



Schjelderup-Ebbe, T., 176, 

184, 185, 189, 191 

Science in general, 15, 20, 90 

Selection, 120, 125, 210, 228 

Sex, 47; origin, 84; relation 

to social dominance, 191, 

194; division of labor, 247, 


Shaftesbury, third Earl of, 24 

Shaw, Gretchen, 92 
Shoemaker, H. H., 185, 191, 

Social, origins, 29, 244, 272, 
274; inertia, 43, 44; appe- 
tite, 44, 47; facilitation, 134, 
172; hierarchy, 175, 207 
Sociology, general, 200 
Spermatozoa, mass protection, 

68; length of life, 83 
Springbok, 108 
Starfish, brittle, 44 
Statistical probability, 54 
Struggle for existence, 26, 51, 

210, 242 
Sully, 236 

Survival value, 32, 49, 133, 
173, 244; of breeding col- 
ony, 111; of social hier- 
archy, 207 

Tadpoles, effect of numbers 
on regeneration, 98 

Temperature, mass protec- 
tion from, 58; effect on 
growth of mice, 102 

Termites, 32, 48, 261, 264, 
265, 271 

Territory, bird, 135; a factor 
in social rank, 193 

Toleration, 43, 44 

Trial-and-error, 44 

Tropism, 246. {See Forced 

Tsetse fly, minimal popula- 
tion, 108 

Ultra-violet, mass protection 

from, 59 
Undercrowding, 50; harmful 

effects, 52; effect on growth, 


Vetulani, T., 99 

War, 210 

Wasps, 262, 264, 265; solitary, 
46; importance of females, 

Welty, J. C, 92, 136, 159 
Wheeler, W. M:, 29, 40, 246, 

259, 262, 264 
Wheeling of bird flocks, 198 
Wilder, Janet, 59 
World Court, 238 
Wright, Sewall, 117, 240